You're already planning on dd "helping", you (why only dd?!) describe your kids as "all grown up" they're absolutely not! As parent to an 18 yr old I can assure you this could very well be the lull before the storm! You're not even properly into puberty yet!!
You haven't as far as I can tell considered how any of the children will really feel about such a decision.
The possibility of the child having health issues or you developing health issues as a natural part of the ageing process - yes many people in their 50's and 60's are fit and healthy and have the energy to be running around after a toddler/primary aged child CONSTANTLY. But many are NOT, are winding down physically, are starting with issues with arthritis, less energy and similar...
Cousins and neighbours kids are not the same as siblings. I have an only not through choice but circumstance, I love dd to bits but young children need CONSTANT entertaining/minding. Babysitting your siblings or cousins little ones is NOT the same experience. You seem to have forgotten the relentlessness of having a baby/young child.
As Larry55 points out if the child isn't healthy (more likely with older parents - not just you but dh too) then that decision to have the child DOESN'T just affect you, you could well be putting your existing children in a very difficult position - I actually know someone in this very situation her parents had her brother when she and her older brother were in their teens, the younger brother has a learning disability, her father died a few years ago and her mums in a home with Alzheimer's because she can't care for them both. The result is she hasn't felt in a position to have a child of her own because she already has too much on her plate. There's also the child themselves - the person I'm referring to has had to have several surgeries as a result of his condition and there are very much physical and pain aspects to his condition.
My other friends who are the children of older parents - and there's several - have not found it to be a particularly positive experience.
They love their families in that 'their my family so of course I love them' way.
BUT
They're not close to any siblings and I've heard many stories of embarrassment at school etc when their parents were mistaken for their grandparents, activities limited because their parents weren't physically fit for them - this has caused real resentment in one family as the dad was a very active, sporty and involved parent with the older kids inc being a sports coach but with the youngest wasn't even up to more than the occasional kick about.
I've also heard of the older kids feeling sidelined in late teens when struggling with exams, uni, first serious relationships etc because their parents focus and time was on the younger one/s.
Then there's your own lives, raising kids is great but it does limit certain things and by delaying reaching a point of freedom - by quite a bit you're really looking at another 20 years at least until your times your own here - you'll be missing out on certain activities which you're likely to feel even more keenly as you'll see your friends with kids the ages of your older 2 going off on holidays just the couple of parents, concerts, weekends away, festivals...
Do you really want to wait until your mid 60's to be able to do all that? If you're even up to it/fancying it at that point?
It could also mean you're not able to be as involved a grandparent to your older ones kids as you'd like.
Sorry, but I suspect this is panic at menopause looming and you've for whatever reasons made your focus the family and you think they don't need you as much...I can assure you they do!
But it's also not wrong to have some time just for you and dh and I think you're underestimating how good that can be.
I can't imagine anything more difficult for a teen doing GCSEs/A-levels than having to live with a baby/toddler especially with disturbed nights!
"Ha I have actually parented 2 children already" but you seem to have forgotten/are ignoring all the tough bits! Sleepless nights, teething, colic, bugs, no freedom/independence, childcare costs (do you even know what they are now?), babysitter costs (not fair to bank on your other kids always doing this - certainly they won't be available once they're at uni/working & left home), child proofing, needing to take time off work every time they're sick (and I can assure you in the current climate this is much harder than it was 12 years ago), potty training...
Are your teens REALLY going to welcome all that when they have their own hormones raging, school stress, friend stress, relationship stresses, exam pressure... They're going to want to have friends and later boyfriends/girlfriends round to stay over, they're not going to want a toddler getting into their private space and things and wrecking their tech...
Do you even have the physical space for a baby? As your older 2 are different sexes that means that really you need a 4 bed place, it wouldn't be fair on either child to have one share with the baby.
Can you afford on a part time wage to put another child through teenhood? (Given the ages of your older 2 I'm also strongly feeling you've NO idea how tough and expensive the teen years can be!) through uni?
ALL your posts are pretty much purely focused on the potential positives (which aren't guaranteed).
How would you cope if you mc? If you were advised that tfmr was something you need to consider? If you needed to go on maternity leave much earlier for health reasons?
I don't think you're being realistic and you're also making a lot of assumptions about how others this will majorly impact but who have little to no voice in the decision will feel. This has real potential to throw a grenade into your currently well functioning family.
"Could it be you're looking ahead and seeing an empty nest which having another child would defer?"
I can definitely see this possibility in how op is posting.
"In reality he's probably never going to live independently so this is our life forever, until the day we die." You've been very brave in your honesty about regretting your decision to have him so I hope you won't be offended when I ask - and who will take care of him when you his parents do pass away?
Quite honestly what I keep thinking is it would do op good to borrow one of those robot babies they use to put teens off procreating for say a month? I wonder how keen she and the dc would be then?