Sparkerparker. I'm extremely sorry you've had horrible gossip said about you. I think this has naturally made you very sympathetic to lady A. I was picked on for a while and bullied and I imagine it must be like that so I really am very sorry for you. I do hope it's all in the past now. I have a feeling though that this example isn't quite the same. I don't think the group apart from the terrible woman and worse husband who wouldn't invite the older husband to an event, the rest don't seem to have said anything bad and they hadn't been in the habit of talking about A - this was a one-off. Also they appear not to know A particularly well, which would lead to natural enquiries about who knows her etc.
When I re-read the OP I realised there is definitely something more here. I am not even sure if the group were "gossiping" in a very unkind way apart from that ignorant and immature person and her even worse husband who didn't invite couple A. That is appalling. But I get the impression that the others were talking in a fairly normal human way. People do talk about people and big differences do get discussed. It's how they are discussed that matters. Until that horrible remark, I was under the impression the other remarks were quite harmless. The OP says "At no point was it 'nasty' as such but comments were made about the age gap" and then mentions the remark which she obviously dislikes.
Lady A's reactions to them all is very powerful almost dramatic. But first, I think the woman at the next table is truly the evil gossip (see my post above). I think she took the strains of the conversation she overheard and eagerly embellished them picking out the one detrimental remark and tittle tattled it readily to A as a "concerned friend". The evil muck-stirring little skunk.
And A? She sounds to me as if she never once stopped to think whether she had been given a fair impression of the overheard conversation. She clearly is able to stick up for herself. I am not sure, but I wonder if she enjoyed the opportunity to tear into people with such power that she could make one at least physically sick. In her position, hearing that people were talking about me the first time, I would have started by asking them what they had said. Then asked them if they denied what I had been told they said. Then, if I felt it were necessary, I would have addressed them about whether this was the right way to treat a friend. Lady A couldn't wait to rip into everyone. No stopping to check the facts for her.
That A was so nasty to a teenager tells us a lot about her. She is not bothered about finding the truth, she would rather believe a tittle tattling gossip. She does not care about the feelings of a child, she would rather bully one. She does not care whether it is fair to pick on a teenager, she would rather enjoy crushing one. She does not care whether it is justified to pick on people one by one and demolish them verbally, she just enjoys ripping into them. Without ascertaining whether they had said anything good or bad or indifferent or nothing at all about her.
I have my suspicions about her. Group finished? I expect so. Is that a bad thing? No, it's just as well.
ChorltonCreamery. Regarding apologising, my apologies! I think now a small bunch of flowers maybe! But keep your head high!!! I really have the impression you have been brow-beaten into seeing yourself as guilty when all you might have done is to say yes he did look as old as a grandfather when you first saw him. And as he has grandchildren older than his children by A, your impression can hardly be taken as unkind or mean. I think you were caught in a situation where people spoke nicely and the truth about A+her DH, and only one person said one awful thing - the not inviting thing. That was not your fault! You are a normal woman. People talk about people. You didn't say anything untrue or unkind. Then real gossip mean minded muck raking shit stirrer trots off to embellish all she has heard and pour it into the willing ear of A.
Forget the flowers! She bloody well can apologise to you for assuming you were saying unkind things about her and she didn't even bother to find out the truth. You did not do anything unkind! you have no need to be upset! you can tell her so!!
I've really go to go to bed - migraine - getting visual disturbances don't want to start being sick!
Please hold your head up high
You didn't stick up fro yourself I think because you described yourself as gossiping which sounds bad. . Think which you did:
gossip = saying nasty things, muck-raking
conversation = discussing facts benignly.
good luck!! 
for you!