I've come a bit late to this thread, in part because I don't really qualify. DD decided quite late in the day to take a gap year, essentially because she wanted to take a breath before starting the next phase of her life. So I still have a teenager at home in what would feel like an endless summer holiday if I was not nagging her not to fritter away what is a great opportunity. I should enjoy it while it lasts!
Those who appear oon the "elderly parents" thread will know I took early retirement a few years back, not long after my father died and it was obvious my mother would need support. DH is still some years away from retirement, and working full time with the demands of both children and parents had meant that my local network of friends had shrivelled. There was a year or so which felt pretty bleak, so I understand what Stonecircle is saying.
My advice is to reach out. There will be rebuffs, and it seems odd to be making the lions share of the effort to renew or create friendships, but it pays off. There seem to be set phases in our lives when we make friends: university; certain jobs; playgroups; primary school run. See this as a new phase. My experience has been that people in a similar position to me are receptive to making new friends and to trying new things. Its just that we got out of practice.
I agree with groups. I am a member of a history walking group which visits different parts of London, and have made a good friend through it. I have also got back in touch with people I have liked in the past, and have found that now we are all over the "busy years" it has been easy to rekindle friendships. Curiously the same has been true for school friends.
And MN has been a surprisingly good place to find friends. I now have a couple of local MN friends who I have known for about four years (had to be local - who else is considering secondary schools in Ealing and Croydon) who are interesting and fun. One of the most important experiences of the last few years was spending a morning with an equally lovely poster, whose mother was diagnosed with dementia at the same place in the same week and who shared my sense of gallows humour. And some great mums of medics guided us through UCAS last year, at a point when DD was recovering from a serious accident, and barely able to get into school, let alone deal with the complex tactics that a medical school application involves. I won't meet everyone, but on-line support from witty and intelligent women is also great.
I am still very tied up with my mother's care and other needs, which is tedious, and it is likely to be this way for a while to come. I will miss DD when she goes, who when things were at their worst used to bake me cakes. But I am happy now that DC are part of my life as adults, and that they no longer rely on me and I don't rely on them.