Aha.
Page 86, Runner's companion; second paragraph under "Not Human":
Shapeshifter characters mature
at the same rate as their parent species,
but have longer lifespans, comparable
to orks.
So, some 'shifters are probably very young by metahuman standards, but not all of them necessarily are.
Just like SOME human shadowrunners are 18 or 20 or 22. But some might be 40-something.
As for emotional maturity? They're NOT HUMAN. Their idea of emotional maturity, or not? Completely different from that of metahumanity.