Yes to these! I think it can also be a combo of belief system and the quality of your actual relationship.
For belief system, if you believe that relationships are about exclusivity, or that unique access/possession is needed to feel special, or that sex is inevitably linked to intimacy, or you are more likely to lose someone if they are free etc etc, then you are far more likely to have negative/ jealous/ insecure feelings when your partner is with somebody else. (Or by extension, in many conventional relationships, if he so much as looks at somebody else!)
You will also feel more jealousy If you have a 'fixed pie' mindset (ie there is only a certain amount of pleasure/admiration to go around, so I need to defend my portion, and someone else's attractiveness makes me feel less attractive by comparison), rather than an abundance mindset (there is no limit to pleasure and admiration - in fact theirs can also increase mine; being around beautiful people makes me feel more beautiful etc )
Then there is also the reality of your relationship. There is no way I would want to do this if I didn't trust my DH, trust that our relationship is a much higher priority than play, trust that he loved and wanted me, trust that he will always come back to me, trust that he would shut it all down if I asked him etc..... that security allows you not to be possessive or jealous... but instead be delighted for him if he has wonderful experiences, with wonderful people.
It's an interesting question about age. My understanding is that new generations are more likely to be experimental and see exclusivity as a rather old fashioned idea! But I may be wrong about that!