You can’t teach an old dog new tricks

This must be one of the oldest proverbial sayings in the language. There are many early citations of it (in A Dialogue conteinyng the nomber in effect of all the Prouerbes in the Englishe tongue, 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.

Other ‘you can’t’ proverbs:

You can’t get blood out of a stone

You can’t have your cake and eat it too

You can’t hold with the hare and run with the hounds

You can’t make a silk purse out of a sow’s ear

See also: the List of Proverbs.