What would happen if someone got a demon slayer mark after they'd already turned 25? Would they die straight away?
According to what the wiki says:
Having the Demon Slayer Mark has its costs. Demon Slayers who have awakened the Demon Slayer Mark are said to die upon reaching the age of 25.
and also it is said:
Kokushibo speculated this to be because the mark exchanged the life span of its user for power.
if the first theory is the right one,
the user will immediately die after awakening the demon slayer mark if he/she is older than 25
but if what Kokushibo said is true,
they can live longer and become older than 25.
side note based on wiki:
Yoriichi was able to defy fate and lived to be 85 years old