Not being funny, but how can you tell that they are young from that? And even if he is, young people can get married too.
I think that it's great for couples to talk about getting married, and he obv likes the idea. Maybe it has come naturally to him but he thought she might not be ready or something.
Anyway, for a wedding you need a venue and a date obv, you need to decide about whether you want to/can get married in a church, you need to decide a guest list, who you want to be your best man, she needs to decide who she wants to be the bridesmaids and she needs to sort out a dress! You'll haveto think about food and seating plans, and music for later! You will also need to think about budgets and who is going to pay for it all. (e.g. traditionally it is the brides family, but of course you may want to pay for it yourself or your parents may agree to pay).
Of course, you and your gf will be able to discuss all of this when you are engaged, there is no need to rush to get everything sorted too quickly, but remember to get the invites asap (and well in advance) when you have decided the venue and the date, so that people can keep the date free!
Hope that helps, and good luck!
