Can my adopted child sponsor me green card

[gpkreddy7]

Hello all, We adopted my sister's son who is now at 17 years of age and he is US citizen. We are here in the US working on H1 visa for long time.
Can my adopted son sponsor green card for us once my son is over 21 years of age? Or is it applicable only for actual parents? Thanks in advance for the suggestions.

[newacct]

If the adoption happened while he was under 16, yes.