I've just gotten married, however I have been engaged before - at 19, to a 27-year-old man.
There are SOME concerns I have with younger marriages, both from my own experience, and from observing young people around me get married.
1. The escapism approach - I can't tell you how many young people (moreso than those slightly older), have the opinion that 'once I'm married, all these problems will magically resolve themselves'. I've heard that kind of approach in regards to sex, family issues, money issues, all sorts of things. I used it myself. This is SUCH a dangerous mindset to have - the idea that marriage/partner will 'save' you from a situation. It is MUCH better to resolve this kind of thing BEFORE you marry/get seriously involved with someone - rather than just rely on marriage or a partner to 'fix' it. I know - I lived this one (fortunately got myself out of the situation before it turned into marriage). You might THINK marriage or a guy/girl will help resolve those problems - usually it has a habit of creeping up on you in marriage, due to learned behaviours. Learn self-control NOW; learn wise money management NOW; learn healthy family relationships NOW - before the wedding.
2. "Wedding" fantasy - so many young girls, especially, seem caught up in the 'getting married to my perfect prince and riding off into the sunset' - and often fall flat on their face when they realise marriage is not all 'butterflies and fairy floss'. Marriage IS hard work - it's a continual exercise. Yes, there is a HECK of a lot of fun to be had in marriage, but it's also pretty gruelling sometimes. If you are someone who has a history of dropping things when it 'all gets too hard' - then I would seriously advise delaying the wedding, until diligence and determination becomes part of your everyday existence.
3. Not knowing yourself/your life goals/your personality - again something I struggled with. Admittedly, a lot of my issues in my last engagement stem from our ages - a 19 year old girl is a LOT different to a 27 year old guy (most of the time)!!! At that point, most of my 'life goals' were shaped, simply because of his age, and his career plans (due to being older). This may not necessarily be a bad thing for everyone, and some 19 year old girls may be strong enough to keep shaping themselves independantly of a partner's goals/desires - but it was for me. I barely even knew who I was when I first started the relationship - and before I knew it, I was very much 'moulded' (as much by myself as by him) by his plans, goals and dreams. It took up until 3 months before the wedding for me to realise that what I was 'trying' to be was someone I never was going to be, or be happy being.
Even now, at 25, I realise how young I am - and it makes me shudder I seriously considered marriage at 19. I can say with all honesty, had I got married at that point, I would either be a shadow of the woman I am now trying to stick with a very unhealthy marriage - or I'd be divorced.
I realise my experiences above do not define EVERY young marriage - but having experienced it myself, and seen others around me do it too many times to count - it is why I hope to encourage my children to wait until they are around 24-25 before starting to think seriously about marriage.
Sasch