There are 14 years between myself and DB1 and 10 years between myself and DB2... and there are 8 years between my DD and my DS.
Growing up, I was essentially an only child in many ways. I remember starting school at 4, and being so proud that DB1 was the one taking me - and I remember similar pride that he'd turned up to my 5th birthday party for my friends (rather than the family one). However, I also remember feeling guilty because DB2 was forced to spend time with me - so it felt more like a punishment for both of us. He has no recollection of this, though, and says that he thoroughly enjoyed sitting through various Disney films in the fleapit cinema with me instead of playing football with his mates, or practising his Judo... 
DB1 and I have always been close. He's the first person I turn to in a crisis, and the first one I want to tell when something brilliant happens. DB2 and I had a fractious relationship up until a couple of months ago when he turned 50 and suddenly realised that life's too short.
As for my children... it's difficult. We're at the stage now where family days out are on an even keel because they're 20 and 12 - but when they were 11 and 3, it was horrendous! My DD ended up traipsing around places geared towards small children, and my DS ended up being dragged (once or twice quite literally) through art galleries. Not to mention that I barely slept when my DS was a baby for fear of him not sleeping and disturbing my DD's rest on a school night.
I have to confess to having felt a little broody when my friends had children - all younger than my youngest - but then I got a dog and the broodiness went
Would I have another now that I'm 40, presuming I'm still capable of doing so? No. But I do understand the powerful urge that consumes you to have another child. Just be aware that big age gaps don't always equal friendship in the long term, and teenagers/young adults still need their parents full attention at times (DB2 and my DD certainly did).