Hi, Joan,
One of the most frustrating things for those of us who have to fly very long distances to start the Camino is that we have to buy a return ticket when we buy our flight across the ocean, which in turn means we have to know when we're going to arrive in Santiago! I hate doing this, and I'm frequently wrong, which means either that I have extra days to walk to Finisterre, or I have to hurry up and hoof it more than I would like. So I'm probably not the best person to give you advice on this. And also, since I'm twice your age,
my gauging might be very different (translate slower) than yours.
But having said that, I usually start walking on Day 3 or 4 of my trip. That is, Day 1, leave US; Day 2 -- arrive in Madrid and take train/bus to starting point, Day 3 -- relax, buy food for walk, get over jet lag, etc. Day 4 -- start walking. If you are going to start in St. Jean, it's worth it, IMO, to spend a day there just enjoying the town -- a little touristy to be sure, but quite scenic. So, if you start walking on May 4, and you want to arrive in Santiago on June 6, that's 32 days of walking if I'm counting right. I think that's just a teeny bit short of the "average" -- I think most people take 33-35 days. If you're reasonably fit and don't want to have any rest days to visit, e.g, Pamplona, Burgos, or Leon, the three main cities the camino passes through, I think it's perfectly do-able, especially given how young you are. I think it would be wonderful to arrive in Santiago on your birthday! Buen camino, Laurie