ACL with very little pain. Its the swelling that comes on afterwords and injuries to other structures that lead to pain and inability to walk. Your knee is unstable, but walking straight on even ground isn't really problem for some. Athletes like John Elway and DeJuan Blair played without ACL's. It's just that the majority of people need them to make athletic moves, cuts, and landing on that foot, so they generally fix most athletes as they aren't likely to function without one. If you tear your ACL at 65 and aren't particularly active, you may not need surgery.