People used to get married very young in the war years and some of them have had long and successful relationships.
Society changes though, so we have different expectations of marriage and engagements. Till death do us part etc,
The two factors you might consider are.
1. We live a lot longer than our grandparents. 80 is the average and growing.
2. At 20 you wont apprecuate that you change as a person, so a good match at 20 doesnt mean you will be a good match at 30. you probably havent developed fully until your 30. Your life but I think its nice to meet and engage with plenty of people before settling down so you know yurself and are sure of what you want.