I don’t think age matters at all really, and the circumstances are what make it reasonable : unreasonable.
However just in terms of gut reaction if I just heard that an aquaintance’s son or daughter lived with them:
20-25 wouldn’t think anything, perfectly usual.
26-30 bit unusual, would assume saving for house and they have a good relationship- lovely.
30-40 more unusual, would wonder if there was backstory eg a divorce or illness.
40+ very unusual, would wonder if perhaps child / parent / other family memeber needed care of some sort.