At what age does a person becomes capable to understand Buddhism?
7 year olds one can go forth into homelessness and  Arahantship can be sustained, younger than that is unheard of.

When I was seven
&amp; newly gone forth,
having conquered with my power
the great powerful serpent,
I was fetching water for my preceptor
from the great lake, Anotatta,
when the Teacher saw me &amp; said:
&quot;Look, Sariputta, at that one,
the young boy coming there,
carrying a pot of water,
well-centered within,
his practices — inspiring;
his bearing — admirable.
He's Anuruddha's novice,
mature in his powers,
made thoroughbred by a thoroughbred,
good by one who is good,
tamed by Anuruddha,
trained by one whose task
is done.
He, having reached the highest peace
&amp; realized the unshakable,
Sumana the novice
wants this:
'Don't let anyone know me.'&quot;

Thag 6.10