"Surely being 51 years old in 1959 is a lot different to being 51 years old over 50 years later, in 2015. "
Um, in the first place GYPSY doesn't take place in either year.
Secondly, the very talented actress being proposed is 66, a far cry from 51 then and now.
Thirdly, I don't care how much Barbra Streisand or Meryl Streep jazzercises, biology is still biology. Though one woman in her 60s just carried implanted embryos to term, it's still not a common practice; nor is there any reason to suppose it will become one. Certainly there's nothing to suggest that Madame Rose (who hasn't $88 to her name) could afford such a procedure and maternity.