What's the reason for having such a system formally in place? RA must be relatively common, enough so for there to be bars catering specifically to such a population.
Do Sweetie and Rumble have RA status because they primarily live with their respective sibling? Is this a older-younger sibling relationship common in Equestrian society? Why does Ruby have RA status when she (presumably) still lives with her mother?
Do you like questions, because I sure do.

RA is a pretty common. Anyone over 13 is eligible, as long as their parents agree to it, and they usually do. It’s considered as a great way to train foals into living in a dog-eat-pony world. There is usually a 13+ bar for every 80 RA holders in a town.

If a child lives primarily with a sibling, then they can apply for an ID with the consent of the sibling.  Sweetie has her’s for the business benefits. Rumble has his for the disability benefits.

Living with a sibling is fairly common in Equestria. Enough of it is around to be recognized as a form of guardianship.

RA does not emancipate you. You still must be living with a guardian figure.

Questions like these make me happy.