My son is 10 weeks old and was born at 36 weeks and 5 days at 4 lbs 15 oz.
He has just begun to smile and coo, literally. But he will only smile right after he wakes up.. He holds his head up, but he's still quite wobbly. I see babies younger than he is in bumbo seats or smiling in tons of pictures.
We used ours at about 2 1/2-3months. You need to take into account his adjusted age. He was born 4 weeks early, so you subtract that from his age. So even though he's 10 weeks old, he's 6 weeks adjusted, which is too early for a bumbo, I think.