If you are looking for stats about the uptake of social media, in the US, by age group, then you should download this report from Pew Research.
I am sure that the data is as good as you can get but I would be a little careful how you apply it to your marketing.
Firstly, be aware that the data refers to internet users, not the total numbers of people in the age group. There is a big difference the older the group.
Secondly, there is no real measure on the level of engagement by age group. I would count as somebody who has most of the social media platforms but I only use a couple with any regularity.
I don't think you will be surprised by any of research findings. Dick Stroud