Sure you can believe that but it's not really true.
Grown adults cheat and divorce each other all the time.
The divorce rate is over 50% of all marriages in the U.S. And I bet all of those couples claimed to have "loved" each other. Love comes and goes as it pleases.

I had a 2 year relationship when I was 16 and had 3 one month relationships when I was 18. So really age doesn't matter. Maturity helps with relationships as you get older but can't really predict when you'll actually start loving someone for real.