How young would I be if I stayed in space for 50 years?

You would be exactly 50 years older. If you were traveling very fast, you might notice people on Earth had grown older than you, but you would be exactly 50 years older.
When you travel very fast and look back on Earth in a telescope, you’ll see everyone there has sped up. Their time is going faster than yours. Your time is going exactly the same.
Here on Earth, looking up at you in a telescope, we’d see you had slowed down. Your time is going slowly, and we here on Earth haven’t changed. Our time is going exactly the same.
It’s relative to who you are, the spaceman or the earthling. Both of them see time traveling the same speed. No change. However, when the spaceman gets back he’ll see everyone got older faster.