open since day one vs opened after 15 years of marriage — different beasts entirely or basically the same thing

ok ive been thinking about this since meeting a 27 yr old couple at our club who have been open since the literal first month of their relationship. they've never been monogamous. they describe ENM as "a feature of our relationship not a phase". meanwhile us, f49 m51, married 22 yrs, came into the LS at year 17 after kids were teens and we had the space and time for it. our entire ENM is shaped by the 17 yrs of monogamy that came before it. trust patterns, jealousy responses, even the way we communicate about play is built on a foundation that was monogamous-coded for almost two decades. talking to the day-one couple was illuminating bc they don't have any of that. they've never had to "open up" — there was nothing closed to open. they don't experience jealousy as a "betrayal" reflex bc they've never associated their partner with sexual exclusivity. they handle scheduling and STI conversations and meeting new partners with this blasé practicality that ours never has. so my question is — are these actually the same ENM or are they fundamentally different lifestyles wearing the same labels. the literature treats them as one thing ("nonmonogamous couples"). but the lived experience seems pretty different. is opening up after 15 yrs basically a separate animal from being open from the start. and do the issues we deal with even apply to the day-one folks, and vice versa. ppl who have done both (or know couples in both situations) — talk to me.