You can't teach an old dog new tricks
Old dogs (and people) learn less well than the young.
This must be one of the oldest proverbial sayings in the language. There are many early citations of it (in Heywood, 1546 etc.) and many of those refer to it as an 'old saying'. The earliest example of it in print is in John Fitzherbert's The boke of husbandry, 1534:
...and he [a shepherd] muste teche his dogge to barke whan he wolde haue hym, to ronne whan he wold haue hym, and to leue ronning whan he wolde haue hym; or els he is not a cunninge shepeherd. The dogge must lerne it, whan he is a whelpe, or els it will not be: for it is harde to make an olde dogge to stoupe.
By 'stoop', Fitzherbert meant 'put its nose to the ground to find a scent', as was the meaning of the verb in the 16th century.
See also: the List of Proverbs.