It means because you are older, you are expecting to make a responsible decision, "know better". An example would be of a 5 year old and a 20 year old hitting a bee hive with a stick. The 5 year old does it because he doesn't know what will happen. the 20 year old knows better than to hit the hive because he is older and more experienced. Therefore, the older person is said to be "old enough to know better"...

It means to old to act like a teenager, usually means a very old person should not act like a youth.