I think Thanos is old too.
Without the infinity stones, I don’t know. Loki apparently knew to go for the head, and he’s a master illusionist. Thanos has great reflexes and is far stronger than Loki. So if Loki can decapitate Thanos, game over for Thanos. If Thanos gets a hold of Loki in the process, game over for Loki.
What I do know is Scarlet Witch would tear them both apart.