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.
Usually a child can be moved to a booster when they are too big for a harnessed carseat, and once they are able to sit properly in a seatbelt. A child should be in some type of booster seat until the adult seat belt fits correctly, generally around 8-12 years old and 4'9 tall (Also see Question 9 below). Q8: My child is over 40 pounds or too tall for his carseat, but isn't mature enough to sit in a seatbelt with a booster.