Next up: student discounts and senior discounts.
I in my 30s, don't have an AARP membership, but have the AARP credit card (it's a good card!). It says it's "designed for AARP members" but doesn't say AARP membership is required. I just left the AARP membership number blank on the application and Chase was cool with that.
So, in practice at least, AARP is open to all, it's just not well known/advertised.
If you don't own a car, the AARP discounts seem to be the same as AAA.