Though difficult to do on the fly w/o software to do it for you - heres a good question for you math guys and gals - Yes its not practical but it teaches you how Age adjusted MAC is calculated ...the long way...:

Using Maplesons equation of AGE MAC



and the following values for MAC40 :

Halothane
0.75
Isoflurane
1.17
Enflurane
1.63
Sevoflurane
1.80
Desflurane
6.6
Nitrous oxide
104

Calculate for a 25 year old vs a 89 year old - chose any agent - show your math....

Edited to add: Original BJA article is here