User blog comment:Iamawesome13/character ages/@comment-1038387-20120804165921/@comment-5188108-20120804201247

Nightwing being 19 in season two just means his birthday falls somewhere between the last time he's referred to as thirteen during season one (does it even get mentioned again after Drop-Zone?) and 31st December - i.e. he turned 14 during season one. 14 + 5 = 19.