PMSL Hester - can relate to that one!
Don't have daughters, only sons, but either way I would say 'not until you're ready'. For some than may be 18, for some it may be 40.
I didn't start having kids till I was nearly 38 and DH was 40. It wasn't planned that way, we envisaged it happening about 5 years earlier but it just didn't. However in the 3 or 4 years before we had kids we enjoyed some great holidays (the sort you can't really do with small children) and a financially secure lifestyle - seems like another world now. So having kids then would have meant missing out on all of that.
I can relate to the grandparents thing - my kids on have one grandparent left now, and that's the one thing that does make me wish I'd done it all 10 years earlier. Not from the childcare POV but because grandparents enrich their grandchildren's lives immensely, and viec versa.
Having said that, a schoolfriend of mine became a grandma at 40! It must be very useful to her grown-up children to have such a young and capable grandma on hand to help but personally I would hate it. On the other hand, I'm unlikely to become a granny at 60 unless both my boys have kids when they're around 20. So I'll probably be too old to be of much use by the time they finally do.
There are pros and cons either way and it depends how you look at it. Biologically about 20-25 is probably right, career-wise it would probably be about 35, but who can say? You'll be shafted by the press whichever you choose.