Excellent information, thank you everyone.

Now I clearly understand:

1. My IRR and NAR (not exactly the same thing and NAR is a hack) will decrease as my avg portfolio age increases. IRR will be less than NAR.

2. By always reinvesting the principal and interest, the avg portfolio age will be less than the actual time which passes due to adding the new notes of zero months. The expected portfolio max avg age will be about 1/2 of the avg terms of the notes. So about 18 months for 36 month term notes.

So one way of estimating my expected long term NAR is to take the average term of my notes, divide by 2 and then use the LC chart

https://www.lendingclub.com/info/statistics-performance.action and find the median point for that avg portfolio age with similar Weighted Avg Interest Rate.

For example, suppose I have all 36 month notes with Weighted Avg Interest Rate of 20%. So, after a long time, like maybe 5 years, my expected NAR would be around 9.4% as taken from the LC chart median value at 18 months. My IRR will be less than the 9.4%, but likely close to it.

Is that a reasonable approach to estimating my expected long term NAR (My IRR will be less than the NAR) ?