I don't think loneliness is inevitable - it depends entirely on your personality & circumstances. If you want company you'll be able to find it by volunteering or joining in with social activities locally. One of our neighbours, an unmarried only child who's now in her late 80s, has always had a wide circle of friends & done a lot of things with them (although as they all get older the number does dwindle of course, sadly).
I'm 66, have DH & grown-up kids; I'm not the outgoing sociable type so I like having their company, occasionally, on my terms
They've all left home - currently 2 are abroad, 1 is distant in UK, 1 is fairly local, but mostly they aren't here. We generally communicate via whatsapp!
A single childless friend of the same age as me, who is the sociable type & is very energetic, has loads of friends, does loads of activities, & isn't ever lonely as far as I can tell. She did have a long-term partner who died much too young - they travelled a lot, & I know she misses him, but that's not a massive factor in her life & she still travels without him. (Also she recently got a rescue dog & is very happy about that!)