they were never in the right setting to truly mature and grow up as an adult. they died while they were still in high school and when they were reborn they never had the chance to experience anything that could help them grow
Mangago scans mistook 23 as 25, i read on legal site arien said their mentality age is 23 (not 25), so they died as 18 years old (which makes more sense since they're obviously died with their school uniform in chapter 1), so far they act as teenagers, sometimes they're logical but sometimes they're childish which perfectly defines a teenager
I thought their mentality should be more mature since their still remember their past lives,but their so childish, their trusting wrong people