This is Anime's biggest Dabi question ever!In fact, his age has not been officially revealed by 'My Hero Academia', but based upon clues taken from both the manga and anime fans suppose that he is now in his early or middle twenties at most.
When the flashbacks show him being a child with Shoto, 16 years old currently, Dabi proves to be a good deal older. A 8~10 years gap probably isn't far off.One can dispute what percentage gap is proper, but if we add that to Shoto's age, Dabi will be somewhere between 24 - 26.
By the way, isn't it amazing that 'My Hero Academia' has such intriguing storylines? Every character is living a life before they come on stage and that attracts all inquiring minds, leading to marathon discussions on case scenarios.