under age: https://www.ssa.gov/pubs/EN-05-10085.pdf

"over age" (i can only assume you mean 150 year olds or whatever, dead people are getting checks): https://apnews.com/article/social-security-payments-deceased...

"future birthdays": assuming you mean that there are records in the database, according to elon and 'BIG BALLS' that are assigned to someone who has yet to be born. However I can find no stories to support this. I expect it's typical elon musk puffery.