Nowhere is it engraved in granite that you have to take a grown child with you. I have a friend, like you whose sons are grown with families of their own, and she travels to all kinds of exotic places on her own and has a ball. There are plenty of tour operators out there who cater to single and older travelers and she usually goes with one of those. Every few years, though, she counts up her accumulated miles (like BigLar) and takes everyone somewhere - usually a cruise.
As for me, I always take my unmarried daughter. Not so much for the company, but because she totes the luggage, fetches coffee or tea, and generally smooths the way for her gimpy old mother. Because I have disabilities I would have trouble on my own, but once the travel bug has bitten, I'd find a way to go.