Well, it's clearly not normal because it bound at the low end. But assuming a normal distribution shape for the high side is probably a reasonable first guess.
Put it like this: I'd say that the average being 29 means that around 80% of founders funded by YC are below 40.
Assuming a normal distribution is always a reasonable first step, but when dealing with small numbers in your distribution you have to be really careful as you can go massively wrong with this assumption.
Put it like this: I'd say that the average being 29 means that around 80% of founders funded by YC are below 40.