Everything adjusts based upon the annuity factors described in detail in the paper by Mike Sabin / Jon Forman here: 

https://papers.ssrn.com/sol3/papers.cfm?abstract_id=2393152

In a nutshell, if a 75 yo is in the same Tontine as a 65 yo, even if they invested the same amount, the 75 yo would receive a higher monthly payment. 

If we have a Tontine of only 75 yo's they would of course all have higher initial payouts because at that age, the capital payout can be more rapid without risking the overall solvency. 

The exact payouts are calculated in an actuarially fair method based upon the member age composition, life expectancy rates and the rate of return of the portfolio as re-calculated every month at the very least.