Gosh 4 months! Although Tristan is nearly 6.5 months...

It really depends on the child. Some premmies suffer from oral adversion from being tubed.
Brendan didnt take to solids. At 16 months he was still on slush and wouldnt take any more than 1-2 meals a day. BUT he had some issues including a severe tongue-tie (corrected at 17 months).
Tristan is slowly taking to it. Once a day about 4-5 spoons (baby spoons lol) and thats about it. Slow and steady wins the race