If I was 8 and told Twitter/Facebook/whayever that I was 15, that would anull my acceptence of its T&Cs, but I would be below the age of criminal responsibility in most (and probably all relevent) jurisdictions so the worst repurcussion is the deletion of an account that I would not have been properly allowed to have in the first place.
If I am 18 and I register with the details that I'm 25... Well, I'm not aware of any reason that this would upset any corporate policies or legal barriers (or switch to 28 and 35, or upwards more, if there's a regime with higher-yet age-of-majority rules). In 'social media', you can register as "Queen Hepititus of Gwondaland" if you wish, and while your stated age (or year-of-birth) is likely going to be restricted to a 'limited' range in a drop-down box, they really don't care if you say your DOB is 1/Jan/1900 (if that's allowed) or even 29/Feb/1900 (if the sanity-checker isn't fussy enough), so long as you're not asking them to take on the corporate responsibility of being responsible for your actions.
And, switchways, the 65yo dirty-old-man playacting the 16yo schoolgirl... Whilstsoever that playacting doesn't breach guidelines, it's not bannable, and whilstsoever it doesn't go into illegalities (as acceptibly interpreted by already complex internationalised caselaw) it also should not be prosecutable in any fair court for the illegalities. The age thing might be contributing factor, but not fraud in and of itself.
I'll tell you now, though, that when I've been asked to give a DOB for various places (not actually tied to anything like online banking, etc) , I have never given my actual DOB. Never outright lied, like makng myself not underage when I really am (much too late for that, anyway... I was old enough to get past most (ostensibly) age-limited resources on the web before ther was a web!), but always moved the date by a given amount just to Stick It To The Man Big Data, should The Man Big Data ever get ahold of it. I have never found a T&C that has any reasonable prohibition against my doing this, so it's all fair game.
edit: Just checked, and this forum has my DOB as 00-00-0000. Forgot to note whether that was DD-MM-YYYY or the stupid way round (though internally I prefer YYYY-MM-DD over both of those), but as it wouldn't matter anyway I'm not going to go back and find out properly.
edit2: Erm. Actually I did go back and check. Despite what I just wrote. And it is YYYY-MM-DD, thus 0000-00-00. Just so you know that I had (not) converted it automatically in my head, anyway, yet I had assumed I hadn't (not) done so. That's for the pedants out there..