"Acting your age" must be the ugliest sentence in the English language. There is no "right" way to act when you're a certain age. We are far too much influenced by media, pop culture and the rest of society telling us that you're less of a person when you don't do this, act like that, dress like this etc. If I don't measure up to the image of what a 20-something should be in someone else's eye, then so be it. I do want I want, or at least try to.
Being an adult just means being biologically fully grown. Emotionally, there isn't an adult on this entire planet.