:up: If anyone uses that 'word', they are presumably language learners wrongly overextending the (generative) rule that '-age' can be added to certain nouns (porter -> porterage, percent -> percentage...), and to certain verbs (marry -> marriage, post -> postage) to make a new abstract noun. Within certain areas of business, there may be a jargon usage; for example, if a business makes number plaques for houses they may find it usuful to invent such a 'word'. If a form asks 'What does your business do?', they may say - if there is a small box for the answer, and single words like 'numbering' or 'numeration' don't do the job (because they're about assigning numbers rather than producing plaques shaped like numbers - NUMBERAGE instead of 'We make house numbers.' I wish they would resist the temptation. It was presumably this sort of laziness that allowed people to coin 'signage' which is now- regrettably - in wide use (OneLook lists 21 dictionaries that admit it).
b
PS Don't be misled by the 600,000-odd Google hits. Google ignores punctuation, so those hits include 'number, age', 'number. Age', ...and so on,