I'd say maybe how people (excluding celebrities) treat their own #birthday (ie. celebrations, procedure, etc.) is a better indicator of their life phase. When you and your friends/relative stop treating it that seriously, it's when you've truly become an adult; and when you and they begin to treat it that seriously again, it's when you've truly grown old.
鄙人认为人们(明星除外)如何看待庆祝生日这回事可能能更好地说明一个人在人生的哪个阶段。一个人及其亲朋好友不再把自己的生日当回事的时候,可以说是真正地成人了;反之,一个人及其亲朋好友重新开始把自己的生日当回事的时候,可以说是真的变老了。