It's wrong to describe unseriousness as "childish." Children take their actions and thoughts much more seriously than adults. The people who are most ridiculous and unserious are old men.
"As the metabolic rate slows down, the 'frames per second' of our perception decrease. This makes the external world appear to be moving faster, which is why years seem to pass more quickly as people age and their thyroid function declines." - Ray Peat