By coincidence, exactly 18.5 weeks with each (although Bryony was just over 3 years ago). With Bryony, i wanted to wait until 6 months, but our HVs were quite militant about starting at 4, and I have friends who started at 14 weeks, so I felt I did well to hold out another 4.5 weeks.
I needn't have bothered as she hated solids and was over a year before things settled down with solids. She was big and wasn't sleeping through but the solids made no difference.
With Ben, the guidelines had changed to 6 mths (not that the HVs necessarily gave that impression) and I was going to wait but he seemed particularly keen to start, so I did. Things went much more smoothly with him from the start. He wasn't sleeping through and again the solids made no difference.