I'm an arbitrary putter-offer of particular "milestones" for my kids. Here's a list of the milestones that I've come up with on the fly. Some make sense... some just are, well... arbitrary!
6 = the age you have to be to chew gum
8 = the age you have to be for us to even consider pierced ears
10 = the age you have to be to learn how to use a knife in the kitchen (and thereby cook me dinner!)
10 = the age you'll be when you can have soda on a more regular basis than "on airplanes only"
14 = the age you have to be to start wearing some makeup
25 = the age you have to be to get married
I find that things keep getting added to the list. Did you have any "rules" like that growing up?