Biology and/or Biochemistry most likely (but i'm not in this space myself). You also want to carefully consider who you want to be your supervisor when you write your masters thesis if you want to become a researcher. 

A useful tip I use when it comes to these kind of career questions is finding the people 1 or 2 steps ahead of you and asking them for advice. 

Are there any biohacking societies or students blogging about anti-aging research that you could reach out to? Finding someone who has gone down the path you want to follow and asking them for advice is very valuable, and they'll likely be happy to give it.