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.
Children should stay in a booster seat until adult seat belts fit correctly, typically when children reach about 4 feet 9 inches in height and are 8 through 12 years of age. Most children will not fit in a seat belt alone until 10 to 11 years of age.