The reason teeth move with age is because as you get older your lower jaw grows forward.
As your lower jaw grows forward it also becomes more narrow at the same time and when your lower jaw becomes more narrow it leads to and causes your lower teeth to overlap and crowd together.
This causes your teeth to shift as you age which is normal.
If you want to keep your teeth straight and prevent the teeth from shifting you can get braces or wear a dental retainer.
But if your teeth are just moving and not getting crooked then you won't need a dental retainer.
It's just a normal part of aging for our teeth to move with age.