At what age do you have to withdraw ROTH funds, or can you keep on trading tax-free in the ROTH after becoming 75 years old and older ?

There are no required minimum withdrawals from a Roth IRA while the account owner is alive. Minimums are required by beneficiaries based on the Regulations for required distributions when death occurs before Required Beginning Date (RBD). RBD for IRAs is the April First in the year following the year age 70 1/2 is attained.

See Chapter 2 IRS PUB 590 Pub 590