Hi belle
It's a tough one. I was 39 when DS was born, and then it took us nearly 3 years and 4 mcs until we finally managed to get a sticky bean (I'm now 30+5). Tbh, I was ready to give up and to move on, trying to come to terms with the fact that DS would be an only one, when I got this BFP.
I originally wanted 3. I originally wanted the 'perfect' age gap of 2 years. Etc etc... All that went out of the window.
I will be 44 in January, DS is 46. I'm the oldest in my baby group and pretty much anywhere with baby-related activities, school etc. And you know what? It doesn't matter one bit. We don't mind, our friends don't mind, and DS most certainly doesn't mind. And neither will the baby. Age is relative. We are fit, healthy, we can cope with anything that life has/is/will be throwing at us, just as any 20- or 30-year old. We do lots of stuff with DS, which we have no plan of stopping, activities, sports, trips, holidays, the lot. I have no desire to join the PTA, but that's not age-related, it's just not my thing
.
Yes, I do worry about DS and DC2 potentially having to deal with the issues that come with having old parents, which has been the main driver for keeping ttc for a sibling - so he has someone else. But who knows? He might not get on with his sibling! I don't get on with my brother, that's just life.
So for now, we will focus on enjoying life with the kids to the max, get them to be independent and self-sufficient as early as possible and try to put measures into place to decrease any burden that might arise when DH and I are old and frail 
Having said all that, and in spite of as sad as the thought makes me - we are almost certainly done with ttc now. Not because of my age (ok, maybe a bit, since it's linked to the main reason), but simply because I cannot face going through the whole ttc process again, after all that has happened to us. The 4 mcs have pushed me/us too far. I want to move on, enjoy life and focus on what I have, and DH agrees (he did comment that he'll be at retirement age by the time DC2 moves out, so I reckon that age plays more of a role for him).
But that's just me! You need to make that decision - you've been through the similar as me, you know what it's like, the heartache, the utter fear, so I guess one decision would be if you are ready to face that again, fight that battle? Forget about age and all that, it's secondary (at least in my opinion!), if it was just down to that, I'd say definitely go for it - you have lots of time. It's all the other sh*t, you know all about that.
Sorry for waffling... But I was in your position for so long, I've asked myself the exact same questions. I'm glad I kept going, but the price was high, and it nearly broke me.