"i'm finding some Jens Peder Rasmussesen's in the 1890 census, yes, but not mine who should be 39 years old."
Okay, well look below at the 1890 census I posted. Name is spelled Jens Peter Rasmussen. (not PeDer)
On your own, without that advice, you can just search for Jens Pe%er Rasmussen.
Now go to ddd.dda.dk and choose the census option.
I use the Danish language version of ddd.dda.dk. Choose it if the options I tell you aren't available on the English version.
Okay now on the main search page, you can select the census year (default is alle aar = all years).
Also select the age (alder) of the person.
Use these options to limit your search when you get too many Jens Peter Rasmussens and it's hard to see the right guy.
You can also select your herred and sogn within Maribo amt to limit your search. Since we already know those for the census posted in this thread - you can go ahead and use them.
Otherwise, it's tricky because a person isn't necessarily living where they were the last time you looked. Pesky ancestors are always moving about.
Keep asking questions if /when you wish. I think you're smart to struggle to learn how to do the research yourself - harder now but it will pay off in the long run.