Are you sure? That page says it's ages 50 and over, which seems like a senior discount. Reading here [1] I see So can you join AARP if you’re Under 50? Yes [...] However, you won’t necessarily get all the benefits available for those over 50. For example, some discounts offered by retailers have age restrictions and require you to be above 50 to take advantage."

[1] https://www.retailmenot.com/blog/why-joining-the-aarp-under-...