Rebecca was very mature for her age, more than I was at 3.
The good thing about this match is at 37, Isaac is still a young buck, but has a degree of life experience under his belt, and being a man, is clearly in control to guide his young wife in a Godly life.
Logistically, Isaac was likely 3, and Rebecca was 4, the Medieval scenario still make perfect sense. People were less coddled then, and children were in fact regarded as miniature adults really.