Madeline went off her milk at about 4 months old - was only taking 4/5oz at each feed. She was also waking at night starving hungry - so I knew it was the right time for her to start having some solids. So at 19 weeks I started her on baby rice at tea time. The week after she was having porridge at breakfast time and baby rice. Now, at 23 weeks, she is on 3 meals a day. I still offer her 4 bottles a day, but she rarely takes more than 5oz at each one, and never more than 7oz. She just seemed to go right off her milk.
Her usual day goes:
6-8am - bottle (between 4-7oz)
9am - baby porridge (a good 6 teaspoons mixed with water)
noon - bottle (4-5 oz)
Tub of homemade, pureed veg
3pm - bottle (4-5oz)
5pm - tub of homemade, pureed veg
Jar of baby pudding
7pm - bottle (3-4oz)
I sometimes top her up with a few oz of milk before we go to bed too, depending on how much milk she has had in the day.
We haven't looked back since we started, and went quickly from just having baby rice at tea time to three meals a day. She took to feeding from a spoon straight away and is now much more contented and sleeps so much better.
I think every single baby is different and there are no hard and fast rules. Some babies are ready for something extra at 14-15 weeks - others may be over 25 weeks old. You will know when the time is right for your LO.