It doesn't really matter how old they are. You can dye your hair with henna at anytime. Don't worry about damaging your hair, henna doesn't damage the hair and the worst that can happen is the colour isn't what you expected. Usually it comes out as an auburn colour. You simply cover the hair in the henna paste, make sure it's saturated. Leave it in for a while and then wash it out. I'd suggest being cautious at first, leave the henna in for only 20 min or so and see how the colour is. If you want more, just dye it again and leave it in for longer this time.
You'll probably want to add things to the water that you mix the henna with, it helps with the colour. I use a mixture of 2 cups of black tea mixed with the juice of 1 lime and 1 teaspoon of cloves. Make as much of that as you need to get the past the consistency you want. You can also add other ingredients to change the final colour more. For example
Strong coffee or espresso - Shifts the colour towards a rich, dark brown
Cloves - brings out a chestnut brown
Cognac - shifts the colour towards reddish orange
Camomile - lightens the colour
Olive Oil - extra conditioning
Keep in mind that obviously henna dyes clothes and skin as well. So be careful