Wake up. Discrimination is not cut and dry, law is not cut and dry, and reality is not cut and dry.
The closest I could find was this: http://www.paulgraham.com/mit.html
But all he really says there is that younger founders have an advantage that they can live on less money.
Try this for example: "Our VC firm does not invest in companies with black founders. We looked at the data, and companies with black-founders have a measurably lower ROI."
Are you still pretty sure?
I think the group who actually get discriminated against, if otherwise just as qualified, are 0-18. That's who I would discriminate against, based on US laws, contracts, etc.
We know from public YC companies that the majority are in their 20s, but there are a lot in their 30s, and several examples in 40+. What we don't know is 1) how many applied in each age group and 2) were they of differing quality other than age.
I'd be inclined to suspect YC gets a lot of the best 20-something entrepreneurs to apply, but doesn't yet get a large percentage of the qualified 35+ year old entrepreneurs to apply. I think that's for three main reasons:
1) a perception that YC is youth-focused
2) diversity of resources and other diversity for older entrepreneurs (IMO, a 25 year old who just did a $50mm exit from his second company has a lot more in common with a 45 year old who has done the same, than either have with a 22yo who has done nothing but college or a 45yo who has worked for the local government in IT his whole life)
3) people get trapped into mortgage/family/etc. and aren't willing to take 3 months off and move to the Bay Area to do YC, even if they think they're willing to do a startup (3 months in the bay area is far from the greatest hardship you'll face...)
If I have missed such a law, please do enlighten me.
As far as I know, I may pass a company directive that we'll only buy stuff from companies run by homosexual mormons born on friday the 13th; and that will be just as legal as those companies that have declared that they will only buy stuff from local companies. I may create an investment fund, stating that it will only invest in companies with Christian (or Islamic) values and it will be just as legal as creating an investment fund, stating that it will not deal with companies allegedly involved in bribery.