What age do I have to be to buy real estate in the US

Well, you have to be old enough to legally sign the mortgage documents, which means you need to be at least 18. You also have to have a decent credit rating and a verifiable source of regular income. That would put the absolute minimum at 18, but in the real world, you'd probably have to be older.

Old enough to sign legal contracts which is usually 18 years of age.

Of legal age, in most states 18, some 19. Otherwise you need a guardian to sign.