Is hairline maturation a long process? How long does it take?

Maturing of a hair line happens as you transition from puberty to young adulthood.  It may take several years.

Hello James.  The male hairline starts to change in puberty, turning from a young boy's low, rounded hairline, gradually moving higher up and assuming a more square shape in the early 20's. In your mid 20's, the corners of the hairline near the temples start to receded a little, giving the hair line a convex shape, that is considered the mature hairline.  As you continue to age, the hairline slowly recedes further back, especially along the temples, giving more pronounced corners.  Eventually, in your 40's, 50's and 60's the hair also starts to thin in the crown (vertex) and top (midscalp).  So in the absence of hereditary, premature hair loss, the hair line continues to change slowly over time, but reaches a more or less stable pattern in your late 20's.In hereditary hair loss, the process is much accelerated depending on many factors, but follows the same pattern.  Because of that it is very difficult, before age 25, how much hair loss you are ultimately going to have and what your stable scalp will look like.  That's why we rarely advise a patient under 25 to undergo a hair transplant procedure.

That may seen almost every young boys almost between 16-23 that have a very rounded hairline without recession in the corners.  Recession in the corners is a sign of mature pattern male frontal hair line.

The hairline matures at any time between 17-29 years of age and it may take a few years to make the transition. See pictures below