Two things you have to change.
First of all you go to

"Settings -> Advanced Settings"

and there are two options:

"Lowest age possible for site members"
and
"Highest age possible for site members"

You change those settings to your needs AND don´t forget to go to

"Settings -> Profile Fields Manager"

and look at

"Max and Min age"

there you put something like this in

'75','14'

if you want 75 as the max age and 14 see more as min age ;)