When birds fly around and can reproduce we call them adult birds. Mammals about the same. Criteria are:
size
secondary sexual characteristics
fertility

but over the last 10 or 20 years peeps have decided that none of these normal criteria apply anymore. the new idea is that brain development is the only relevant factor. and not even brain development, only one type of brain cell means anything in their model :-D

The human brain reaches maximum size around age 13. 

Any objective assessment finds this fact relevant :-D 

The idea that all mammals become adults through a process called puberty EXCEPT humans who have to wait over a decade post puberty for some obscure and mysterious brain cells to grow an extra 0.3% in order to become adults is completely ridiculous.