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.
It depends on the proportions of your child's body, the shape of the vehicle seat, and where the seat belts are attached to the vehicle. Most kids need to ride in a booster seat from about age four until ages 10 to 12 years.