I think it honestly depends on the people and how much effort and love they continually put into the relationship. I don't think that you wake up every day fabuliously in love with your SO.....but I think that you learn what makes each other tick and continually work to keep the spark alive in your marriage and to compromise and learn how to make each other happy.
My sister Kimmy got married when she was 18 and her husband was 19 - they just celebrated 14 years together last month
They are still going strong and I can definitely see how much they love each other in the way they talk and act. I've learned soo much from them and continually ask them for advice!
My oldest sister Monica and her husband and been married for 22 years (23 in November). They also got married when they were 18 & 19. And they are still doing wonderful.
My mom on the other hand got married when she was 17 and later divorced her first husband when he came back from war hooked on drugs and wouldn't get help. She went on to marry 2 more times both ending in divorce. (AMy sister's Monica and Kimmy and I all have the same mother but different dad's. My sister Debbie and I have the same dad but different mom's).
I know that her marriages didn't work out not just because they weren't right but also because of her personality and the way she never gives in or budges. I adore and love my mother- but she is probably the worst person when it comes to relationships because she thinks that all men are bad. It has taken my sisters and I years to try and convience her that there are good men out there (our husbands for one) and that she really should have a different mindset. She is set in her ways though and I don't believe she will ever remarry or be happy in a relationship if she does marry again unless she is willing to learn to compromise and trust another person. I love her, so i don't mean any of that in a mean way- just saying she's my example of what not to do in a marriage I suppose. She spent years trying to convience me all men were evil and other nonsense- but thankfully I had a great man (DH) prove me otherwise
So in my view- i think that people can get married young and have loving lasting relationships - but i think all marraiges take work and people have to understand that. You just can't give up when you're frusterated or angry- that's not what it's about.
( I'm 23 and DH is 32 (9 yrs older than me to the day lol). I don't think age matters so much as your willingness to work together as a team and constantly try to show each other how you care for them.)