Keeping your avatar looking relatively young gives many more clothing options!
But why focus only on age? Why not body shape too? Rarely in SL do I see an avatar who might be described as overweight, let alone one who is positively fat. The contrast with RL is stark. Where in SL are the droopy bits and cellulite?!
SL allows us, if we choose, to escape our RL selves and that is a good thing. For those whose conscience makes them uncomfortable pretending to be in SL something that they are not in RL, there is always the option of adding a RL pic to your profile.