Mandibular growth is noted as being "statistically significant for the age periods of 16 to 18 years and 18 to 20 years".
https://www.ajodo.org/article/S0889-5406(05)80052-6/pdf
It is clear that the mandible can grow reasonably after the age of 16, and certainly after 18. In fact, the jaws, especially the mandible typically grow until the age of about 25. Depending on how much growth hormone your body is producing, which varies by the individual, and depending if growth is guided/stimulated, positive mandibular growth seems very realistic.
Here is an interesting example on the Orthotropics reddit page:
More than likely his total body posture improved.
The mandible grows, yet he still possesses the same curvature as before.
Mandibular growth is noted as being "statistically significant for the age periods of 16 to 18 years and 18 to 20 years".
https://www.ajodo.org/article/S0889-5406(05)80052-6/pdf
It is clear that the mandible can grow reasonably after the age of 16, and certainly after 18. In fact, the jaws, especially the mandible typically grow until the age of about 25. Depending on how much growth hormone your body is producing, which varies by the individual, and depending if growth is guided/stimulated, positive mandibular growth seems very realistic.
Here is an interesting example on the Orthotropics reddit page:
There is a definite improvement, however, what you've stated about a males jaw growing way into the mid 20's is false based on current research. The lower jaw actually stops growing around 18-21 max for males. Also, the growth that can be experienced during 18-20 is bout 2 times less than the growth at 16, so one shouldn't expect much mandibular growth at those ages, unless we're talking guidance induced by fixed appliance.
The changes you are referring to are changes induced by remodeling, which can take place at any age.Tthe lower jaw can be moved forward and backward at any age according to some very credible people on here, including an ortho who goes by the name of SUGR.