I had a surprise baby at 40.
His sister and brother were 22 years and 17 years old when he was born. He is the best thing that ever happened to me!
"Starting over again" has been great and lots of fun.
My husband was very worried about the increased likelihood of chromosome anomalies, so I had the blood test at ten weeks which made him feel less worried. But the risk of Downs or any other syndrome is actually very low when you're 40. It's only when you're nearing 45 that the likelihood is really increasing - and even then the large majority of babies are just fine.
As previous posters have said, practical things can usually be fixed. My grandmother got married at 38 in 1940, shortly after German bombs had destroyed the town she lived in, and both she and her fiancé (my grandfather) lost everything they owned. My mother was born when my grandmother was 42, and her brother when she was 43. At that time my grandfather was 58 years old. They were lovely parents for my mother and her siblings, she had a very happy childhood.
I find that being pregnant and having a baby at 40 was no more difficult than it was when I was 22. It did take longer for me to feel my good old self again after the birth, but being older has its advantages, too! For instance I'm much better at prioritising and using my energy where it's actually important, and at taking one day at a time and not wear myself out by thinking of everything I have to hurry up and do.
We were shocked when we found out I was pregnant, but if I hadn't had this baby I would truly have regretted it. It might have been my last chance to experience this miracle.
So I recommend having a baby at 40, it's great.
It is of course your decision if you want to keep this baby - but I don't think you should let your age weigh very heavily. 40 is not really that old.