I guess in the same vein the Spirit Guides spend half their time in a trance, the stargazers stay up all night stargazing and are too worn out to travel in the day and the alchemist spends most of his time stoked up on the latest distillates (the Imperial Alchemist has learnt better with age)
I guess I view mapmove 2 as a sort of default capability for a healthy, motivated individual (of any age - there are enough elderly hikers out there who I can't keep up with). Units with move 1 are being slowed down by something, be it infirmity, personality, size, weight or just disinclination to hurry.
I completely agree with Evil Dave that some afflictions should reduce map move (probably to a minimum of 1). That would be a great thing to mod in.
And special afflictions for old age would be cool (if perhaps impossible). Arthritis could reduce map movement and increase encumbrance in cold provinces, for instance.