First things first: their similarities.
Their hair is styled in very similar ways. Dabi's bangs fall in nearly exactly the same way Touya's fall. You can look at any character we've seen as a child in the series and can tell their hair doesn't change very much, if at all, as they age
Next are their eyes. Yes, Touya's are much rounder. Of course they are. He looks pretty young in that picture, but I'll get to that later. Manga illustrators portray youthfulness and children with large, round eyes. Still, you can see the corners of Touya's eyes point downward like Dabi's do. For comparison, look at young Bakugo and him now. His eye shaped changed drastically as he got older
My friend was yelling at me early this morning when I was looking at different character's ears and how Natsuo and Touya's are the same shape. Ear shape is based on genetics and Dabi, Natsuo, and Touya have rather round ears. See best boi Natsu here:
Their ears are different from Shouto's and Enji's:
Next is Touya's rather round jawline. Again, children have round faces and that changes as they grow, but Dabi's jawline is still rather round.
(And to be fair, Izuku and Bakugou still have fairly round faces at the ages of 14-15)
There was a theory that I read not too long ago about what may have happened to Touya. I agreed with it completely and the arguments they made against their own theory. Why didn't Shouto really react when Natsu mentioned him? It's been confirmed that Fuyumi is the second oldest sibling. Natsuo is four years younger than her and Shouto is seven years younger. Shouto was raised separately from his siblings and with Touya being the eldest, he probably saw him the least and wouldn't remember him well, if he even saw him at all. Enji did believe that since Touya couldn't use his quirk without seriously harming himself, he was useless. The Todoroki estate is large, he could have been kept far away from the rest of the kids most of the time.
Now to address his age. That is difficult. When we saw Touya, Fuyumi and Natsuo playing in the yard, Touya was undoubtedly the smallest. He has thin arms compared to his siblings and was the shortest despite being the eldest as we learned here:
I'd like to go back to the new picture of Touya. At first glance, you'd think him a young child, elementary school age based on his features. Upon looking closer, I noticed he is indeed wearing a school uniform. In Japan, elementary school students uniforms do not look like that. The coats typically only have a single large button, three larger buttons or have double breasted buttons. Most elementary school uniforms do not button at the neck. What he's wearing looks more like a gakuran uniform with gold buttons which is what we typically see a boy wearing in middle or high school in anime and manga. Sakamoto wears one, the boys in Yu Yu Hakusho wear them and so on. It's a more traditional uniform from what I understand but are a bit more common than the more Western Catholic inspired uniforms we see, which is what the UA uniform is. This implies that he was at least in middle school and could have been 11 at the youngest and 14 at the oldest at the time that picture was taken. Going from that picture, in the shot of them playing, Shouto was 5, Natsuo 9, Fuyumi 13 and Touya 13 if he is Fuyumi's twin and 16 at the oldest. Of course, it is possible Touya went to a private elementary school where young kids also wear this uniform, but I might doubt Enji would send his "failure" children to a school of a higher caliber.
Quirks manifest by the age of four, but the first quirk manifested in a newborn baby. It's not impossible, just unlikely for that to happen too often. Enji stated that at the age of only 20, he felt that he would not be able to surpass All Might. We know he is 46 and Fuyumi is 23. He had Fuyumi when he was about 23 or 24 years old, which means he could have had Touya when he was 20 and shortly after he married Rei. Approximately three years difference between the two siblings, just enough time to see what his first child's quirk would be, if it sufficed and if not, planning for another baby. So, Touya could have been 13 when he "died". Practically everything about Dabi is unknown, but a police report stated he appears to be in his early to mid-20's.
Shouto is 15. Natsuo is 19. Fuyumi is 23. Touya/Dabi would be 26 at the oldest.
What about his height? Again, Enji said Touya was weak. He had his mother's weak constitution. Natsuo said they can't withstand heat very well, most likely because he and Fuyumi inherited her ice quirk but most likely a little weaker than hers. If Touya was born premature, he would be even weaker to fire in addition to having his mother's constitution. My sister is premature and despite being several years older, I am taller. Height is just one thing that can be effected by being born premature, but that isn't always a life long situation. A study by the University of Auckland had stated that by late adolescence, those born premature often catch up to the average height. What age exactly is late adolescence? Between 18 and 20.
Dabi is a lanky boi. He's taller than all of the females in the show and taller than or the same height as a majority of the males, which proves he is about average height. Dabi is taller than Shigaraki. That is clear even if Shigaraki was standing straight, but not by much. Shigaraki is 5'9 or 175 centimeters, and Shouto is 5'9 and 1/4, 176 cm. Hawks is 172 cm or 5'7 and 3/4, but I'll just bump him up to 5'8. Bakugo is also 5'8 and Dabi is definitely at least a couple of inches taller than him.
Natsuo is a bit taller, standing at 5'11 and is definitely taller than Bakugo. He's still taller than his dear big brother, but not by much anymore.
It's all coming together. We're in the endgame now, folks
