All over the age of 18+ (as required by LPSG)
I would frequently start getting erections anytime I was talking with a woman who I had any interest in whatsoever. Just talking, getting up the nerve to ask her out, etc, and I would start to chub up. Back in the era of tighty-whities, any obvious signs of an erection were obscured. My first date, and first kiss was while I was in college, and while making out, there was nothing stopping getting a full blown hard-on. While the relationship with that first girlfriend never turned sexual, she certainly must have been fully aware of my arousal while we were kissing and I was caressing her bountiful breasts.
In later relationships that were sexual, I had to use mind control to keep my carnal thoughts under control when we were around family and friends, or out at a club or bar. I'd allow myself to get aroused, but never a full erection, as it would have been obvious to everyone around.
When alone with a woman with whom I'm in a relationship, I allow my erections to happen and do nothing at all to hide or suppress them. More often than not, it has led to having some fun, even if that wasn't the plan of the moment!