Board Thread:Naruto Gaiden/@comment-26539840-20150629040037/@comment-945885-20150701071637

His exact age isn't known, but I'd estimate him to be about 33, give or take a year.

The Last takes place in the Winter two years after chapter Number 699 and Naruto's birthday is in October, so Naruto should be 19 in it.

Chapter Number 700 takes place "over 10 years" after The Last according to the official timeline. It's safe to assume that Boruto and his peers are around 12 years old, so it's actually been 13 or 14 years, most likely.