The human brain is made up of different regions that control different areas of the person. Some areas look at the cognitive or thinking fuctions while others control the emotions and base instincts, and so on. Emotionl regulation is now viewed as being related to the links between the prefrontal cortex of the brain and the subcortical areas that include the amygdala and hippocampus.

Speaking very generally, the prefrontal cortex (loosely speaking, the cognitive brain) reaches maturity more slowly than the other areas of the brain. Neuroscientists have indicated an age of around 25, possibly even higher till 30. There is likely a lot of individual differences due to nurture, genes, the environment, etc., but the point is that emotion regulation reaches maturity later than the milestones of 18 and 21 years old. 

This can explain how many young adults and teens struggle to manage their emotions and with impulse control. They and others could wonder why this is so, think they should be old enough to know better, and end up judging themselves or being judged harshly. 

May this perspective help the critics, both inside and outside, to soften their voices, and bring compassion instead to the youths struggling with emotional stability. 

Reference

Tottenham N. (2017). The brain's emotional development. Cerebrum : the Dana forum on brain science, 2017, cer-08-17.