The age at which a persons voice is fully developed is around 17 years old.
Your voice begins to change around puberty which can be as early as age 10 but usually it doesn't happen until age 12 or 13.
That is for both boys and girls.
Boys may experience a voice change sooner than some girls though.
But it can vary.
The average age for a boy's voice to change is around 12 years old.
A boy's voice begins to change from their child voice to adult voice around 12 years old and they usually have a harder time talking because of the voice cracking and sounding different.
This can be embarrassing for kids especially in school but it's a normal thing that all boys go through and eventually the cracking of the voice will stop once the boy's voice gets to where it should be.
The voice is usually fully changed by age age 15 to 16 depending on the boy.
When a boy goes through puberty that is when the boy's voice begins to change and it can begin as early as age 10 but most commonly it happens around 12 years old.