What is the ruling of Islamic law on using substances that change hair color to black or red, and using substances to straighten curly hair? And does the ruling differ between young people and the elderly?

It is permissible to change hair color to anything other than black and to use substances to soften it, whether for young or old, provided there is no harm and the substance is pure and permissible. It is not permissible to change hair color to pure black for both men and women, due to the Prophet's (peace be upon him) saying: "Change this gray hair, but avoid black."