In the (barely) over 50 dating category, are women still "expected" to have sex with the guy on/following the 3rd date or the 3rd week for that matter?
To be honest, I have no idea where the whole Third Date Rule came from or who invented it. Three is an arbitrary number with no real significance. What's more important, I think, is that it's clear after the first few dates that the interest (and attraction) is mutual. Sex is not the only way to convey interest.
Physical intimacy is important as it's what delineates between platonic and romantic relationships. So it's vital to display some level of physicality as you get to know someone. That could include anything from a light touch on the arm to intercourse. It's all dependent on your (and their) comfort level.
What truly matters is that you make your interest known. That could be by initiating and planning a date, paying for a date, sending flirty texts, etc. As long as you're putting in tangible effort, someone will wait for sex. It's when people feel like they're being tested or forced to jump through hoops that they bail after just a few dates and no sex. To reiterate that point, it's not the lack of sex that is the problem. It's when one person feels they're being used that creates a disconnect. That goes both ways. Both women and men are wary of being taken for a fool.
Could a man lose interest in a woman who is not willing to sleep within after a certain number of dates? Sure, if he isn't sure where he stands with her. If he believes she's sincere, he'll wait it out. He might get sex elsewhere during that time, but if his interest in her goes beyond the surface, he'll keep dating that woman until she's ready to be intimate. Is there risk involved with waiting to sleep with someone? Sure, but no more of a risk than going on a series of dates and growing to care about someone. Sex or no sex if you like someone, you get invested. Attachment doesn't require sexual activity to be real. The chance of being hurt exists regardless of physical intimacy.
The risk of getting hurt or rejected is always present. That's why sex is not a bartering chip or benchmark of how someone truly feels about you. Have sex or don't. What matters is why you choose one or the other and if that choice is authentic or fear-based. The former is more likely to lead to a relationship. The latter won't.