What Mexxi said - why does anyone have to "propose" when you already own a house together, live together, have been together several years and have told one another you intend to marry one another.
As far as reason is concerned you are already engaged, as you have agreed you will eventually marry one another - that's all being engaged actually is, a proposal is just frilly romance novel decoration on top of a preexisting commitment.
Stop dropping irritating hints and tell him you want him to be the one who decides whether to turn the life support off if you're in a car crash, not your mum and dad, and you believe he feels the same, and it is important to you to set a date for being legally one another's next of kin. If he won't, ask why not. How can you be "too young" for the legal commitment if you already live together and own property together and both believe in marriage?
Have a very honest conversation and get to the bottom of it, be absolutely clear you want a date set and won't be strung along with romantic, fluffy, out of date, silly male controlled proposal etiquette.
If he doesn't want to set a date and flounces you know where you stand and can set about separating your assets. Don't be strung along or drop hints or play games, its both a bit pathetic and probably very annoying and also gives him the idea that he has power and you have to wait and be girly-grateful.