To answer the main question first, it probably was true at one point, back when people lived in smaller towns or villages with simple lives, they were regarded as wise because they had the most life experience with every day problems..

Is it true now though in the context that you mean? is it fuck. Nowadays we are reading every day about international crisis', wars, political negotiations.. And there's no reason granny should have a deeper understanding of these issues than anyone else. True, older people have experienced more, but they always experienced it from their own perspective.. A 17 year old idiot is going to view an issue differently to a 17 year old genius.. 40 years later they could have been either one of those people, the only way to find out is to talk to them and have them explain their argument. If they can't and choose to rest solely on "wise old age" then they aren't worth the time.

Having said all that.. The "too young to understand" thing does have some ring of truth to it. There are certain things you need to live through for some time, namely the political system, to fully shape your view. I'm guilty of thinking the same thing at times, like when I see the "junior wing" of a political party on the news waving banners and essentially reading off a party statement verbatim when asked for an opinion. Just a feeling of "we'll see what happens now that you're paying attention", the things and ideas that you're drawn to when first engaging in politics may not hold up to scrutiny as you begin to read about them each and every day. It's a naive stage that everyone must go through, in the beginning you come in waving facts and figures with a general sense of faith in the system, but then you slowly pick up on the other unspoken but very important dynamics at play.