I've got DS1 age 15 and then a younger two, 2 and 5. DS1 has a different dad but my DH is like his dad - he's been stepdad since he was 2 and they have a good relationship.
It's been so so brilliant and such a privilege going back to the baby/toddler stage.
I think if they are keen for a sibling then they will likely be excited. And I would probably let them know at the 12 week scan. Or any time after 10 weeks confirming a heartbeat. We had a couple of early miscarriages when DS1 was about 8 and he actually said he'd have preferred us to have waited to tell him until we were past that risk point (which I think we did for the second one).
What we've done is sort of include him more on the adult side - obviously, he's not an adult, but things like we told him the name when nobody else knew so it could be our family secret, he also had input into DS3's name - though we ultimately chose it. We also revealed the pregnancy to him a couple of weeks before announcing it publically.
The hard parts have been trying to reconcile preteen/teen need for attention and connection in the evenings whereas the young ones tend to need you all day and in the evening when you get them off to bed you think "Finally I can breathe/relax!" but you can't because that's when teenagers emerge from their cave 
This I think takes some careful management - I have also struggled with my energy levels due to personal issues which didn't help here. DH has been very supportive with sleep and taking over baby/toddler duties when I need to rest which has been invaluable.
We have also been totally chaotic WRT mealtimes over the last 5 years. I think again this is due to personal factors and wouldn't happen with everyone. But basically, DS1 had always been fussy, and by 10 he could make his own dinner if he didn't like what we had made, so we ended up in a pattern of making 3-5 different meals all individually. I am trying to streamline this now with an expectation that DH cooks for everyone once a week, DS1 cooks for everyone once a week, I cook for everyone twice a week and the other 3 days we all have our own individual thing as we have been doing (there's also a potential DH & DS2 cook together but due to timings this doesn't happen every week).
It is tricky here because the younger ones tend to need to eat dinner at 4/5pm, whereas DS1 sometimes doesn't get home from school until later than this and DH might not be home from work. Probably what we need to do is coordinate two mealtimes, baby dinner at 5ish and then adult dinner at 8ish and then I just get everyone to confirm which one they are going to eat and cater accordingly. But it feels a bit too difficult to do this right now. I will have to figure that out.
In terms of shared family activities, it's kind of gone like this -
When DS1 was sort of 10-12ish and DS2 was 0-2ish, we mainly did things based around what DS1 would like or things that all three of us would enjoy seeing DS2 explore that DS1 could also join in with, without a fear of being seen as "babyish" - this was really lovely and kind of extended that childhood bit a little bit.
Unfortunately covid hit when they were 2 and 12 so that got in the way a bit, though in a way was nice as we had lots of family walks etc. Over the next year, we had DS3 and of course the restrictions waned after that. So once they were coming up to 14,4 and 1, DS1 was basically used to staying at home and entertaining himself while we took the smaller DCs off to run errands or visit parks or do whatever entertainment was allowed but didn't allow extras, plus he was more inclined to want to go out with his friends than hang around with us, which was fine. So now he tends to go off with his friends or stay at home, and we spend more time with the younger children. It tends to be holidays and big days out that we plan where he'll come with us. But in general, I think this age gap is great in terms of balancing their needs because you can focus on the older one(s) when the little ones are at the age that they are entertained just by a new environment, random sticks etc and then by the time the younger ones need something a bit more focused on them, the older one is more independent.
Everyone went on about "Ooh, free babysitter!" but in reality, he took DS2 to the park on their own a few times when DS3 was tiny, and he has looked after them in the evening literally once - we don't expect him to babysit regularly, though he is happy to offer for a special occasion, I think as it means he doesn't feel he has to get us a present He did end up getting pushed into the tiniest room, but he quite understands that the little ones need space to play whereas, as he puts it, he doesn't need a huge room because he just sleeps, does homework and goes on his computer/phone in there. The only issue with this has been space for sleepovers.