When dealing with a headache, try boiling one cup of apple cider vinegar and one cup of water in a pot. Pour into a heat proof bowl and put on the table or other safe place that you can lean over the bowl and cover your head with a towel. Inhale the steam for a few minutes to bring some relief.
The warning on mine says not to use prior to 4 months of age. My son was in his Jolly Jumper from 3 months. The directions of use are that if they can confidently hold up their head then they are fine.
Then, they can move to a booster seat. Ages 4 to 8: Children must remain in a booster seat in the back seat of a vehicle until they are at least 8 years old or 57 inches tall. Once they reach that age or height, they can use the regular adult seat belts.