I think it goes further than that - post 35 (post 30 really) there is a lot more pressure to shift from casual dating to a relationship at a faster pace.
Life experience talking here too, it isn't just "being upfront", it's also being willing to move at a faster pace.
Intention and reality are two different things - although I can't say I ever managed casual dating for 2 years! Usually most folks are in a relationship after 1.