I agree, if you're going to the trouble of modelling then a stochastic matrix (markov chain) is a much better model. You can model it as a markov chain where ea

I agree, if you're going to the trouble of modelling then a stochastic matrix (markov chain) is a much better model. You can model it as a markov chain where each month there's a probability of moving to age + 1 or a probability of becoming 'ex-sub' (general population). You also have a transition from 'general population' to 'month 1' to represent new sign-ups. That has the advantage that you can still find the stationary distribution (steady-state) of the chain without simulation and it doesn't treat churn as being the same for each subscriber age. (It still has it time-homogeneous for a given age of course).