My mum had a 3rd DC at 39, DSis and I were 14 & 12. Although DBro wasn't planned, the whole family (including grandparents, aunties and uncles) got so much joy out of him. DM really enjoyed having him in a way that she didn't get the chance when managing with a toddler and baby before.
Secretly, I think DM also loved that she had an excuse never to work full time (she had been considering it before then) and then by the time he was older in his teens, well, it was nearly retirement time 
I loved having a baby brother, though DSis at age 14 was less keen - she felt the responsibility very much (and to be fair we did too much babysitting as my DM often worked evenings and weekends and DF was more hands-off in the style of a 1970's dad, not a 1990's one). This is still a major source of resentment for DSis for various reasons.
I would say though that generally several girls/women I know with much younger brothers or sisters - there is a really special love there that I can spot a mile-off.
DBro effectively had 3 mums. He and I had a bit of a brother-sister relationship, but not really, he was more of an only child.
Big downside is that it was just him and parents at home by the time he was in his teens, and he was only 22 when we lost DF.
If I was you I would definately do it, but I wouldn't wait too long.