I did when my oldest was around 6 weeks. He ate all the time so it helped him feel fuller at night. Nowadays they say it's unsafe, but my son and every baby whom I've cared for over the years whose parents wanted rice in their child's bottles turned out just fine. It definitely helps, just make sure you don't mix it too thick or cut the nipple too big and you'll be fine :)

And I honestly can't remember how old he was when I started

I did, and it definitely helped him sleep better. But when he got a little older the rice cereal made him constipated so I switched to multigrain and it helped him poo.

I did at 4 months, my baby didn't start sleeping through the night though until 6 months when I started sleep training.

I did with both of my babies at age 4moths !! It does helps ! Just don't put too much