21 is a year or two ahead of the typical graduate school entry age (the bright ones who go straight from BSc/BA to PhD, that is). You are either precocious, an early entry child or moved from abroad and skipped ahead in the school system.
Have you graduated yet? Three years is a very short grad degree, unless you did a masters or are in a non-science program.
As for your dating concerns, 19 year olds are barely out of high school. Its no wonder you'd find them immature. Try mingling with grad students from other departments to find someone like-minded. Find some seminars outside your dept that might be interesting & chat them up afterward at the coffee table. Or date women who are already in the professional workplace after uni after their first degree (e.g. nurse, engineer, accountant).
Second thoughts can generally be amended with judicious action; injudicious actions can seldom be recovered with second thoughts.
--Cyteen by C.J.Cherryh