Hello! If you're referring to real life I think under today's social standards you should marry someone when you think you are ready and to be ready within today's standards comes with lots and lots of expectations.
One of the top reasons for divorce can be financial instability so make sure you two have lots of savings ready for the future together. There are a couple of ways to check if you two will be financially stable after marriage. The first is definitely the amount of savings you have. It will also depend on how expensive the wedding will be, and how much are you two willing to spend on the honeymoon. If you two plan on having children early or even buy a house, not rent! A lot of planning has to be done. Many things will start to sink in after marriage that are necessities to have such as life insurance for your child and partner. At least, I would like to be someone that makes sure my partner stands with both feet on land.
The second thing you have to ask yourself is how well do you know your partner? I understand that not all marriages have the privilege to answer this question. Be it a shotgun wedding or something that happens while you're drunk or high in Vegas. However, I hope this is a committed and serious relationship in question. And in that case, it would be wise to stay with your partner for a good while, at least until it feels like you can settle with them and you're used to them. Surprisingly, there are some couples that have done nothing but date for their entire lives and when they marry and finally move in together, they discover something distasteful and it may lead to disagreements.
Which leads me to my final point. Do you two agree with each other? Are you two ready to take the next step together? It's not only a question you ask yourself but also your partner. Many things you do down the line will be beautiful as it may most likely involve them. Anything that happens to them will be associated with you as well. Have you two discussed about the non-financial standpoints of marriage? What kind of marriage do you two want? If you are unsure about all these points, I think it's alright to sit down with your partner and discuss things openly with them. You will be spending the rest of your life with them anyway so might as well.
Anyway, if you were referring to srp just head to the townhall when both characters are 20+ to officiate the wedding with a government worker. A Shinto wedding can also be arranged if you approach a shrine worker(me)!