is 30 the the new twenty?

for the past 10  years i  have heard people say that 30 is the new twenty and 40 is the new 30 but is it really? and if this is true, should it be?
longevity is certainly a factor of the the "new" decade definitions.  when people lived shorter lives it was more important to be more serious about life sooner. with shorter life spans it was more important to have children in one's early twenties because having a child at 40 would mean one had even less time to see them grown up.
though its nice to have extended adolescence  im not sure its necessarily ideal. i think it would be nice to have a spouse and a family earlier in life so one could be more settled earlier and have longer to spend in the more "settled " stage of life.
then again, no matter how "settled" life can seem, there are always curve balls . someone who gets married young could also be divorced or widowed young.
i think that it is good that people are given a longer acceptable time to be young and unsettled these days than they did fifty years ago.it eases the pressure.
i realize that in frum society people are still expected to marry young, but i guess since i am more modern orthodox, there have always been a lot of people around who haven't married young.
as much as frum society thinks that they aren't effected by secular society they are still effected by it. this is why there are a lot of frum people who are in extended young adulthood.