visiting yong ding tolou, do we need to pay for entrance fee? how much? any free entrance for above aged 70 ?

Yes, you need to have to pay for most of the Tulou.
As I know, there are many Earth Towers in Yongding with different entrance fare.
For the Tulou in Hongkeng, the entrance fare is around 90RMB per adult, which is most visited by tourists.
For the discount for senior people, it is suggested to ask the staff in the scenic spot directly with your passport.