Still going at 3 yrs while pregnant with #2. We're cutting down feeds at the moment as it's become painful for me in the last few weeks, but I expect to be tandem nursing come November. We'll see! I don't think there's an age children should be weaned by. It's all a matter of social context; in Mongolia they consider the longer the better and have a saying that all the best wrestlers breastfed until age 7...
Breastfeeding absolutely made TTC#2 difficult. I didn't have periods until 19 months, then they were irregular for nearly a year. We TTC from 14 months for a year with no luck, then stopped trying due to work issues. I fell pregnant immediately when we stopped preventing 6 months later. I was in a position where I could be patient and allow nature to decide when the next baby should come, and am delighted with the timing in the end. However, had I been in a hurry (e.g. if I were older) I'd have had to seriously consider weaning at around 2 if not before.