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.
Colorado Law requires children to be in boosters (or other appropriate car seats) until they are 8 years old. However, best practice and the safest recommendation is to keep children in a booster seat until they are about 4-feet 9-inches tall (which is the average 12 year old) and can pass the 5 Step Test*, for seat belts.