The Cobbler's children have no shoes

The oldest version of a proverb with a similar meaning (i.e. one doesn't always benefit from the product of their trade) is to be found in the Bible, Luke 4:23

Physician, heal thyself. Whatsoever we have heard done at Capernaum, do also here in thine own country.

The Oxford Dictionary of Proverbs gives - as Jim already stated - 1546 for the first mention of

But who is wurs shod, than the shoemakers wyfe, With shops full of newe shapen shoes all hir lyfe?
[1546 J. Heywood Dialogue of Proverbs i. xi. E1V]

With reference to the cobbler this proverb exists also in French :

Les cordonniers sont les plus mal chaussés.

with a first quote by Montaigne : Quand nous veoyons un homme mal chaussé, nous disons que ce n'est pas merveille s'il est chaussetier in his Essais (which he started writing in 1572, i.e. after first mention in English)

In German:

Die Kinder des Schusters haben die schlechtesten Schuhe.

Similar proverbs are found in Spanish (En casa de herrero, cuchillo de palo "In a blacksmith's home, knives are wooden"). In Chinese "the lady who sells fans fans herself with her hands", in Arabic, "at the potter's house water is served in a broken jug". ^*

^* I don't speak these languages and I haven't found the original sayings, I can only quote from a book on proverbs I have in French.

There are many variations on this cobbler proverb, but here is the oldest I can find, reported to be from John Heywood's Proverbs part i chapter xi, published in 1546 (and quoted later by Richard Graves in The Spiritual Quixote, published in 1773):

But who is wurs shod, than the shoemakers wyfe, With shops full of newe shapen shoes all hir lyfe?

A couple of similar quotes are:

A plumber's house always has a dripping tap.

A blacksmith's home only has wooden spoons.

I have seen interpretations of this proverb that focus on a possible altruistic or industrious dimension to its meaning. That, for example, the shoemaker is too busy to attend to his children. perhaps overworked to make even more basic ends meet.

I'm less sure and would rather concur with the above meaning. It's a great phrase as it succinctly describes a conundrum of the human psyche.