Advancement by higher education
If 4 sessions of 10 weeks each are assumed per year (with make-up work for failed research) wizard advancement through higher learning could be predicted. Costs could be higher, but the 100 GP or $1,000/week rate would be double the minimum for research (this is the ideal rate, meaning 18 INT, advancement would be about 3/4 at INT 14 and 1/2 at INT 9).
Level 1 assumed to be the start point (High school equivalent/18)
Next year would complete basic apprenticeship resulting in level 4 (Certificate degree/19)
Year 2 would get character to level 5 (undergraduate degree/20)
It takes 2 more years to get to level 6 (Bachelor’s degree/22)
Another 2 more years are needed for level 7 (Master’s degree/24)
Takes 3 years more to reach level 8 (Doctorate with internship/27)
Takes 4 and a half more years to reach level 9 (Professor – becomes an alchemist at this point/32)
At level 9 the character has spent 13.5 years in additional studies after reaching level 1 (at $40,000/year for college that amounts to a $540,000 education – if ant INT 9 character made it this far it would be 27 years/about 45 for human, and $1,080,000).
Alchemist level wizards advance slowly taking about 3 years to reach level 10 (35), 4 years to reach level 11 (39), and 11 more years for each level after 11 (50+).
This would make a 12th level wizard 61, 13th level 72, 14th level 83, 15th level 94, 16th level 105, 17th level 116, and an 18th level wizard 127 years old (would be capable of casting wish, but would have needed to find a way to offset aging by this point).