that was my thought exactly. Additionally, if forehead growth compounds we're looking at a much shorter window.
Using the compounding growth formula that we all know and love (PERT), and assuming his forehead was originally 2 inches tall and increased by 4 inches (to a total of 6 inches) over the course of 20 years, we get the following equation to solve for the rate of change.
2 inches * e ^ (r * 20 years) = 6 inches
e ^ (r * 20 years) = 3
r * 20 years = log(3)
r = log(3) / 20 years
r = ~1.0986... / 20 years
r = ~0.0549... per 20 years
If we substitute this r value back into our original PERT equation and solve for our time to reach the distance to orbit (62 miles = 3,928,320 inches from thread OP) we get:
2 inches * e ^ (0.0549... * t) = 3,928,320 inches
e ^ (0.0549... * t) = 1,964,160
0.0549... * t = log(1,964,160)
t = log(1,964,160) / 0.0549...
t = 263.797 "time units"
keeping in mind that our original r value was per 20 years, we multiply out and I believe our final result is ~5275.94 years of growth if we assume compounding growth