I'm happy to see that grammar is being seen as important enough to be taught in English schools ( http://www.bbc.co.uk/news/magazine-22403731 ) again. I think. At least it might improve some people's dating prospects, according to the article.
In the above article appears:
The Idler magazine's Bad Grammar Awards recently named and shamed a letter by academics for saying that the national curriculum demanded "too much, too young" - thus confusing an adjective and an adverb.
There is a reasonable amount of evidence online that the expression is acceptable, however, and one could argue that "too much, too young" could be considered a shortened (and punchier) form of "[they are (being) expected to do] too much [(-/,) while they are still] too young".
'They married young' is quite widely used and sounds acceptable to me - of course, this expression can fairly be analysed as 'link-like verb + adjective' without having to consider young as a flat adverb.
When expressions achieve idiomatic status, of course, traditional grammar is no longer a controlling factor - by and large coordinates a preposition and an adjective.
Is the Idler's criticism justified here?