There are no simple answers to the many questions you have posed here as every person is an indevidual. I will take sex first. (No pun intended)
This is a basic instinct, no different from eating; a necessary physical act which can be very pleasurable. Without it humans would not exist. There are no rules to follow as each person has different needs. Ten minutes once every six months for one person is right, while another needs two hours out of every 24 and this is also right. Unfortunately, many problems can be caused if two people who wish to be together do not have the same needs. Some people can love each other and never have sex.

The same applies to preferences. Some people believe there should only be one man & one woman involved, while others want same gender or multiple partners. As long as the people involved are adults, and in agreement, nothing is wrong which is done in private. Like any other 'skill', experience is the key. While there are novices aged 60, some at that age would find a partner of 30 quite boring.

Romance is something different, a very pleasant experience, which may or may not lead to sex.
Then we have love. True love is, for me, the spiritual side of a relationship.

Growing old gracefully is O.K. but growing old disgracefully is much more fun.