An npc asking you to move in is pretty rare now, it used to happen to my sims a lot more in the past.
Also they will never ask you to go steady or propose, unless you mod the game to allow it to be autonomous, for those options to show up on your sim they have to be at extremely irresistible, is it not showing up even then? and is your sim his only romantic interest? perhaps he already has a girlfriend hence no option.
Having played this challenge there is a little leeway, she wishes to marry him and in order to do so they have to go steady, the 'go steady with' wish does exist as my self sim has rolled it at least twice for her other half in a couple of games but it's super rare, so to marry them you will have to propose going steady first even if she hasn't wished for it.
As for life span even on normal it's not 90 days and you die, one of my dynasty spares lived for 110 days, if she is 90 just how old is the child? you are calling it a baby but surely it is older than baby by now, I know teens can live alone as I do that all the time, not sure about children, I think it might be game over in that case.
