I am now 25 years old, want to learn boxing, sparring, or taekwondo which is more appropriate

This depends on personal preference, taekwondo on the body flexibility requirements of higher, mainly practicing leg, you 25-year-old man pressure ligament will be more painful, the effect is not very good (I was 17 years old to start practicing taekwondo).

Sparring is better in real combat. The hands and feet can be practiced

Boxing should be practiced reaction sensitivity and upper limb muscles!

All in all, in a word, it depends on personal preference!