I'm a 35 year founder of a small company in San Francisco. I had never heard of Venmo until recently. The dividing line among my team is around 30 years old. Below 30 -> use Venmo all the time. Above 30 -> never heard of it.
This divide isn't really an age thing IMHO, its an attitude about money (related to age) thing. In my early-mid 20s most of my friends were just starting out and counting every dollar. Taking the hit for an entire round of drinks or the pizza delivery was painful enough that you asked for everyone to chip in or at the very least had a careful mental accounting of whose turn it was to buy next (and if they somehow weaseled out if it, that was a strain on the friendship- and I am sure most have cut some moochers out of their life).
This may vary by your group and where they at in life, but by my late 20s, those things mattered a lot less. If I buy someone a beer, I just hope they will get me back at some point and if not, then I bought them a beer- no big deal-enjoy! (well enjoy, unless this is the fifth time in a row I have done this and you haven't gotten me back).
I am in my mid-late 30s, and I have Venmo. I just tend to not use it very often, only for fairly big ticket items- concert/event tickets, someone buys a gift for someone else that we all chip in on, a restaurant gives a hard time about taking multiple credit cards, etc.
