Dht has no effect on brain development. People born without the ability to make 5ar (precursor to dht) have shown no impacted cognitive ability. Dht is used mainly in developing male characters. The majority of that of which are completed by 16.
As far as not being physically or mentally mature, what are you basing that on?
A quick google search tells me that DHT is responsible for the biological characteristics of males, including a deeper voice, body hair, and increased muscle mass. You think a 16 year old has developed all of those?
