The usefulness of the clichéd phrase day and age is questionable, but it makes sense as a redundant way of saying era or just age. Day in age, on the other hand, is a nonsensical eggcorn derived from a mishearing of day and age.
Day in age is most common in speech, but it occasionally appears in writing—for example:
In this day in age, what prevents an attorney from making an oral argument or even examining a witness over Skype? [Minnesota Lawyer]
In this day in age, humans still migrate to places of greater opportunity – but that’s by choice. [Washington Post]
And these writers spell the phrase correctly:
You almost gain nothing by talking about things really early in this day and age. [Los Angeles Times]
Hopefully, in this day and age, everyone sees taniwha in the same fairy tale realm as goblins, elves and fairies. [New Zealand Herald]
Part of the controversy, inevitably perhaps, concerns Miss Monroe, who, in this day and age of diminished glamour, remains an indubitable star. [Guardian]