There's a huge amount of polling data here:

Trends in party affiliation among demographic groups

The topic you are interested in has this heading and is about
Quote:
A wide – and growing – generational divide in partisanship


Some of the numbers are quite startling. Like 35% versus 17% identification with Democratic vrs Republican Parties among Millennials. You have to get all the way to the Silent Generation (1928-1945) to see a slight Republican advantage. But if you look at the earlier gender preferences, you can see that women are way more Democratic than Republican and women tend to outlive men by several years. This means the overall voting preferences of the Silents will change as more men die.