It's the way of this SMF forum, showing of age is a completely optional thing really that is only shown when visiting a profile. If people want to have ridiculous numbers for their age or displayed as their "character" (Undead, vampire or whatever they can come up with), it gives them the option if they want to be displayed this way.
Internet thing really, if you want to give people this freedom you know many will put whatever in there, don't really find it any worse having a user lying about their age being 17 instead of 14 for example, whilst something over the top like that you can easily see like a joke or "derp" thing.