I don't know where my personal line is - all I know is that it's likely to change! When R was born, if I was asked to put an age on it, I'd have said "maybe when she can walk over and ask me for it". But really, she does that now - OK, so she crawls into my lap and pulls my breast out rather than walking and politely verbalising her request - but now I see no difference, and I think it'll be really sweet when she asks. I was with a friend's 21 month old last week who pointed and said "boobs" (she's still having an evening BF) and her mum had to explain that yes they were boobs, but that she couldn't have any. Actually, she'd have been welcome - that's something else that has disappeared now I am an actual BFer - any feeling that it would be odd to feel another woman's child. But that's another thread!
I just don't know when I think you "should" stop - but I'm certain that my view will get later as R gets older.
I don't think you can stop a baby from growing up by breastfeeding the, longer than this society is used to. R is 14 months, and seems quite grown up among her peers, in terms of her independence, wandering off without me, not clinging, willingness to try new things, eating skills (though not quantity) - but her cutlery waving surpasses her friends. So I don't think I'm holding her back for any selfish reasons <shrugs>