"Keeping" & fully functional? Two totally different questions. Plus through in the age factor, and add in the brain factor(s) of how much of a horn-dog you are to start with.
I know how badly my wife wants me to be "functional". We did have some great times in bed as well as other places

During my On/Off low-dose HRT brain rescues in the past, I always needed to stop due to "The Prime Directive", otherwise known as being a normal(ish) guy. When things started to take a hit below the belt, I stopped HRT since their primary job was finished. Now on full feminizing dosages of both E and an AA for 8 years it has been all but completely dead, shrinking and shriveling up for some time now. Only starting to breath a little when I start fantasizing too much about them not there and with a b/f
YMMV