I have seen that some people use materials to dye their hair different colours, be it black, red, etc., and also using something else to make curly hair straight. It is permissible to do such things, and does the ruling apply equally to young and old?

Answer:

There is nothing wrong with changing the hair colour and dyeing it any colour, except black. There is also nothing wrong with using something to make curly hair straight. This ruling applies to young and old alike, so long as no damage results from doing this and the material used is pure (taahir) and something that is allowed in Islam. However, dyeing the hair pure black is not allowed for men or women, because the Prophet (peace and blessings of Allaah be upon him) said: “Change this grey hair, but avoid black.” (Fataawa al-Lajnah al-Daa’imah, 5/168).

Changing hair colour and “relaxing” the hair
Fataawa al-Lajnah al-Daa’imah, 5/168

http://www.islam-qa.com/en/ref/1194/