I could just simply say "No. Your friend is a moron and needs to be beaten with a copy of Lego Star Wars as soon as humanly possible." But instead, I'm going to have fun with this. I am going to take some time right now to have a little fun, and by fun I mean I am about to say some stuff that will make people want to either shake my hand or rip out my left lung. Let's compare a few "kiddie" games with a few "mature" games, shall we? (Note, I am guessing your friend simply means games tarted at an adult audience, ie rated M for Mature, when he uses the term "mature" because any logical human being who actually IS mature is also aware that something doesn't necessarily have to be targeted at them and their tastes to be good.)
First lets warm up with a bit of a curb stomp battle. Let's compare Sonic Colors, a game clearly targeted at a younger audience complete with an E rating and, as the title sorta implies, lots of shiny colors, with SEGA's attempt at a more "mature" take on the same franchise with Shadow the Hedgehog. If you already can't see where this is heading, you clearly have been living under a God Damn PLANT until recently. As any once wide eyed, hope filled Sonic fan (myself included) can tell you, good ol' ShTH was about as fun as has having one's eyeballs forcibly removed with a nail clipper. Seriously, it's been over half a decade now and there are still people I know who freakin' shudder every time they see that god damn Super Sonic recolor. Sure the game TIRED to be more "mature" but unfortunately blood, guns, and and some curse words do not make for a good game, especially when said game is basically a crappy, broken mess of a mod slapped on top of a mediocre at BEST game (Sonic Heroes). Meanwhile you have last year's Sonic Colors. Is it mature? Nope, it's definitely made for the younger audience in mind, but it's so much fun that it doesn't matter. The 3D to 2.5D gameplay blend works to keep the game from being "push right/forward and jump sometimes to WIN!" and I walked away from the game feeling like my $50 were well spent, unlike Shadow where the god damn BEANIE that came free with the game was the better product, and that damn thing was too small for me..... and I don't wear beanies.... oh well, at least the Sonic Riders T-shirt (which is also of infinitely better quality then the game it was made to promote. Seriously it's a pretty comfy shirt.) fits me well.
"Oh, but the Sonic series was having trouble anyways" you might say, well then how about another little comparison. I know, let's take a little trip back to 2008, ooooh what an interesting year that was, and this time lets compare a "kiddie" game with a game that wasn't well.... crap on a disc. In the "kiddie" corner, we have a little known game called Tales of Vesperia, you know, that one Xbox 360 installment of that JRPG series that isn't Final Fantasy, but no one played it anyways because JRPGs, especially ones with beautifully colorful graphics, have Japanese Cooties (It's like normal cooties but with tentacles <3). Representing more mature games is (FLAME SHIELDS RAISED) Fable 2, released the same year for the same system. Now, don't get me wrong, I had fun with Fable 2, but I only played it twice. Once as sort of "Doing my own thing based on how I would react" yada etc, then I played again a second time as the unholiest evil spawn from Hell. Once I did that though, I really never felt a desire to go back and play again. Why? Well, despite all of it's "mature" content there really isn't much there. The combat gets dull quickly, the storyline is pretty basic, and it suffers from pretty much every problem found in "nonlinar" Western RPGs (I obviously use the term nonlinear loosely here because well... beyond the "moral choices" and being able to fart at people, there isn't much "open endedness" to be had) Also worth noting in terms of mature content is, of course, the sex..... or well... poorly voice acted clips of people pretending to have sex, which frankly only serves to make the game less engaging and more of the perfect example of why mature content can land you a spot in "So bad it's HILARIOUS" territory. It also doesn't help that frankly the charaters in the game are either not engaging at all or are hardly ever present, which not only makes the plot even less interesting but just goes to further disengage me. Seriously Peter? Why should I want to bother marrying a bloody SIM? Meanwhile ToV is filled with a pretty good size cast of enjoyable charaters, including one of the greatest protagonists in gaming ever (Try to deny it, call him out for looking like a girl, whatever. Your protests will be ignored because the sentence for insulting Yuri is..... decapitation. Seriously though I love his deadpan snarker sense of humor backed up by complete badassiness). Is the game realistic? No, not at all. Is it mature? No.... and yes. I say no because well... it's a T rated game so blood, violence, land language are kept to a minimum but the storyline does dive into some pretty mature stuff, and does include scenes involving massive wars, political struggles, blurred lines between good and evil, and suicide (If you know the scene I refer to with the last one, Go ahead and take a second to get a tissue. I cried at that scene as well.) Now let's compare this with the more mature Fable 2 for a second, what mature themes did it explo......... Oh... sister got killed, that IS kinda dark I guess but is there anything else? Okay my actions have consequences.... okay sure, but my character gaining a couple of pounds from munchies isn't a big deal really, and I could care less about what the SIMs walking around me think. Oh, I guess there's something about chaos and the world and order or some..... okay, we're not really going to dive any deeper then a jar of baby food, are we? .... And now my dog just broke the jar and now I have smashed carrots all over my shirt. Thanks a lot Peter, you over-promising, lier jerkface. I will also take a moment to note that ToV is probably one of my favorite video games EVER that I have played through several times while Fable 2 is pretty much a "meh was good while it lasted" piece. Also ToV is now available for a damn worth-it price on Games on Demand on Xbox Live. If you haven't played it, you should. Take it from someone who HATES Final Fantasy.
Okay, one last comparison (Since is your still reading this you have WAY to much time and probably should be doing something more productive then reading a random dude's post in a rather silly topic) This time let's just pick to games from each category that have NO RELATION to each other. Let's see, for the "kiddie" side, let's take take the colorful, lighthearted and well beloved Mario series, Super Mario Bros to be specific and compare it tooooo.....hmmmm.....oh I know, DAIKATANA!...... What? Why are you looking at me like that? Okay fine, let's go with DMZ: North Korea. Don't remember it? Oh no problem, it was just a little known First Person Shooter, complete with a mature content fueled M rating, delivered by Vivendi, you know the guys who owned Blizzard before being assimilated into the Bor... I mean Activision. So why havn't heard of it, or if you had why you don't remember it? Because it SUCKED. The enemy AI made Fable 2's SIMs look like an army of Leonardo Da Vincis, the story was complete garbage, the graphics looked like something most grade schoolers would be embarrassed to put in a portfolio, and the gameplay was about as much fun as having those nail clippers I mentioned earlier being used to remove the rest of your internal organs, and the person using them is Yamcha from Dragonball...... Evolution. And that is why you don't remeber it, a game from 2007, when you can recall a game from 1985 just fine, because the 2007 game SUCKED and the 1985 game is a beloved classic REGARDLESS of any "kiddieness" present.
Now, if you were paying attention through all of that, you probably picked up an something. In all three of those comparisons my focused was directed at the actual QUALITY OF THE CONTENT, as oppose to the "maturity" of it. Just like with a book, or a movie, or ANYTHING if it's good, the target audience means NOTHING. All it means is that said media was created with a certain audience in mind, but in no way is anyone outside of said audience not allowed to enjoy it as well. Just ask fans of the new My Little Pony series, or fans of Shinryaku! Ika Musume, or fans of .... well.... ANYTHING. Even stuff that isn't targeted at kids isn't labeled as good or bad just based off of the target audience it's made for, not even the target audience judges based off of that, though certainly they may be more accepting of it.
You, my good sir, go on enjoying your Ratchet and Clank like the good man that you are, and if your "friend" decides to rant about the sin of a teenager enjoying a "kid's" game, feel free to call him a grade F moron. Now if you excuse me I have to go pay my respects to Mister Whitehourse (If you got that without missing a beat then congrats, you are a geek).
tl;dr
No. Your friend is a moron and needs to be beaten with a copy of Lego Star Wars as soon as humanly possible.