I think is normal that they treat her like a kid. Someone on their early 20’s is often seen as a kid by the 30+,40+ that the others are, specially if you consider the power thing, with experience and such. So yeah, she might be 25 on Infinity War, 24 on Civil War and 22 on AOU, if this timeline is correct.

She is young, and yes, inexperience plays into the way the Avengers treat her. But the writers have said it’s also a matter of fear-born condescension. You can see it in the argument Steve and Tony have over the pens. When coming to a head about what Steve feels is internment and Tony feels is due caution, Tony defaults into dehumanizing Wanda, Steve defaults into infantilizing her.