In proper English it's grammatically incorrect to use "real" as an adverb. If you do it at any language exam it'll be a mistake.

My understanding is that the difference is more geographical and than generational. Substitution of adjectives for adverbs has been a feature of American English for a long, long time. This is not a 'youth' thing. In fact, unless I'm very much mistaken, AE speakers of all generations could say "You did real good". I'd imagine that it's not impossible that a well-educated young person who's learnt how to 'talk proper' (sic) might even be embarrassed to hear their less educated parents or grandparents speak in this way. This is not generally a feature of British English. If an English person were to say "You did real good", we'd wonder why they were trying to speak like an American.

I am not really sure if I belong to the 'older generation', but suspect they would think it was sloppy. But.. it depends where you live. More explanation here: https://en.oxforddictionaries.com/definition/real