Choose one. 1. One time payment of $500,000 tax free 2. De-age by 20 years 3. Stop aging for 25 years

Straightforward offer: a one time payment of 500k, aging back 20 years or stop aging for 25 years.

Option 2 will de-age your whole body back 20 years (you'll keep your memories/life experiences of course).

Option 3 will have you stop physically aging for the next 25 years.