Me and DH had ours before 30, and they have left now - left for uni at 18 and never returned, but only live 20 miles away.
We are mortgage free with good pension plans, and decent savings, and can retire and 55. (Around 5 years time.)
If we had had another kid or 2 in our 40s (when our other kids were young teens,) the 'new' one(s) would only be primary school age, and we would probably be a lot less well off, and unable to retire until 66-67.
I couldn't think of anything worse to be honest.
Love our kids to bits, but been there, done that, and have been exploring pastures new over this past 4 or 5 years, and busy looking for a world cruise next year, and looking to buy our first ever brand new car in March 2020. (a 20 plate obvs,) which we will be paying for outright.
And as I said, we are mortgage free (for 4-5 years now,) and are retiring at 55.
Could never do this if we'd had babies in our 40s.
And as a few posters have said, it's all very well saying 'I am fit and healthy at 42-43 and can outrun my 22 year old colleagues at work' and so on...' As someone said, MOST people (in their early to mid 40s,) are fit and healthy and feel youthful.'
However, 10 years down the line, you'll be in your early to mid 50s with a junior-school aged child, and all the shit that goes with that; lots of responsibility and expense and hard work.. And this is the age when you start to feel shagged, tired, weary, wanting to nap more, menopausal, and often having to deal with elderly parents... All this with a junior school age kid.
Then by your LATE 50s, they'll be at senior school, and you'll have school politics/exams/A levels, all the expense of uniforms, school trips, all the study materials etc, in your late 50s! And in your 60s you will have to contend with putting them through uni... And all the faff and expense there!!!
At the point in your life when you would prefer to be retiring and having long lie-ins, evenings out at the pub, socialising/hobby groups, and booking long cruises and holidays with your partner (or friends,) you won't be able to, coz you'll be having to work til you're 70. And you will have shag-all to show for it.
I wouldn't do it. No way. AND, as a few people have said @Shybairns why are you doing this because your new boyfriend wants a baby? What if he leaves you, and you have your 2 to deal with, and another one under your feet til you're in your 60s... at LEAST.
DON'T DO IT! 
I know a few people will take offence at my post, but I am merely posting my views and opinions. And like everyone else, I am entitled to them.. If you already have kids (and had them before you were 30 say,) I cannot see the point of having more in your 40s to be honest; and I really cannot understand why people do!