I don't know when this question sidetracked into a debate about the appropriate age to marry..
As for the cat, I think it's weird..
3 years is a long time. I wouldn't do any planning until you guys are ready to start setting dates. In my experience, you do the most growing/ changing between 18 - 21. Your mindset is going to change a lot over the next few years.
Cheers,
Chris.