I think it may depend on how old you are, and what the "older" is older than. I knew what I wanted in someone from the time I was, maybe, late teens. If I think of the people I found appealing/attractive earlier than that - yuk! I can't even believe I thought some of those people were appealing. Once I was in my late teens, though, I pretty much knew what I wanted and haven't changed how I've thought about that since then (and I have grown kids and didn't have them all that young, either - so you can guess the math).
I think some young people are more mature at eighteen than some others are, though. Then again, some forty- or fifty- year olds don't really know what they want either. But, I think if a person is basically as mature as most other people his age, then I think we pretty much know exactly what we want fairly young (but, again, not mid-teens or earlier).
Life events/circumstances can, I think, change what someone wants. For example, the person who wants one kind of spouse and finds herself a young widow may be changed, and may want a different kind of person if there's a next time around. Or, if someone marries someone who turned out to be violent, maybe that person will look for someone who seems completely different if they marry again. Those are life-changing/thinking-changing type situations/events, though. I think, maybe, those are what can change what a person looks for - not necessarily just "collecting birthdays".
If you think about it, if you have the right values and the right kind of self-esteem there's really not reason why a person would change what s/he looks for in someone else. Sometimes what can make SOME young people change their thinking is that they were valuing the wrong thing(s) in the first place and/or that they didn't have the self-esteem to know what they could/should expect in someone else.
The one thing that has changed is how old the guys are that I'd find attractive (for me - not for someone else). It works out kind of well, as far as I'm concerned.
BUT, obviously - just my opinion.