Is 20 too young to be engaged

I'm 20 years old , junior in college && I've recently accept a proposal bfrom my boyfriend of 5 years. My grandmother says that I'm too young to marry but I didn't plan on actually having the wedding until I graduated undergrad so I continue focus on school && not have planning a wedding as a distraction && he is ok with that. The poll answers would not change my mind because either way we wil be engaged I was just curious.