The Links are canonically children. Trying to argue otherwise is a waste of time. They are treated like children by canonical adults, and they are abducted by Stone Arrghus in the Village of the Blue Maiden. However, "children" includes a wide age range. Himekawa likely didn't have a specific age in mind for them and generally wanted them to appear like tweens. Using some evidence from Nintendo interviews and game/merchandise canon, I will attempt to argue that the Links are 12 years old. This argument is not made for any reason other than my hatred of NSFW works about the Links.
This argument hinges on certain tenets that can be disputed. I am going to assume that the manga Links are supposed to be the same age as the Links in FSA. I will also assume that the Links in FSA are the same age as Toon Link from Wind Waker (more on this later). Lastly, I will assume that the ages stated by certain sources take precedent over other sources. I will then go on to argue why, in my eyes, 12 years old is the most plausible age for the manga Links given their circumstances and behavior.
I begin my argument by bringing into question the age of Toon Link in Wind Waker. Why Wind Waker!Toon Link? Well, many assets from Wind Waker are present in FSA's Hyrulean Adventure and Navi Trackers modes, and FSA was made soon after Wind Waker. Wind Waker was also Toon Link's introduction, so his age was more-or-less established in that game. Additionally, the Navi Trackers mode in FSA uses the exact same model for Toon Link as the one in Wind Waker, and we see Links in all four colors in that mode. I concede that the green Link in this mode is likely supposed to be the Link from Wind Waker, as he seems to recognize Tetra and her pirate crew, but we cannot ignore that other Links appear in the mode as well that match the colors in Hyrulean Adventure. In addition, these Links that appear similar to Wind Waker!Toon Link appear the exact same as the Links in Hyrulean Adventure when shown on GBA screens. It is a safe assumption, in my mind, to assume that the Links in Navi Trackers are supposed to be the same age as the Links in Hyrulean Adventure.
Toon Link's age in Wind Waker is, in game, stated to be the same age as when the Hero of Time left on his adventure as a child. According to Shigeru Miyamoto, the Hero of Time was 9 years old as a child. However, the official Nintendo Power guide for Ocarina of Time states that child Link is 10 years old. The adult Link amiibo states that he is 17 years old, implying that he was 10 years old before the seven-year timeskip after pulling the Master Sword. Taken altogether, this means child Link is either 9 or 10 years old, with 10 years old being endorsed by two sources. (If you want to be really crazy, you could say child Link is at least 11 years old because he would be at least 18 years old after the timeskip, which is the minimum age of adulthood in modern Western culture.)
However, information on Wind Waker!Toon Link specifically seems to offer another possible age. Though it is stated in canon that he is the same age as child Link, multiple sources assert that he is 12 years old: the official Nintendo Power Wind Waker guide, Zelda Box, Encyclopedia, Toon Link's trophy in Super Smash Bros Brawl, and Toon Link's Super Smash Bros amiibo. I believe this discrepancy may simply be due to the citizens of Outset Island forgetting child Link's exact age over the years.
Where does this leave us? According to game canon, Wind Waker!Toon Link is the same age as child Link. If we are to believe Shigeru Miyamoto (a credible source), this means Toon Link is 9 years old. If we are to believe the Nintendo Power Ocarina of Time guide and the adult Link Amiibo, Toon Link is 10 years old. If you want to abide by the rule that adult Link must be at least 18 years old, then Toon Link is at least 11 years old. I believe that 10 years old is the most credible age for child Link (and Toon Link, if he is the same age as child Link), as this is the age implied by the newest source (the adult Link amiibo) as well as the published Nintendo Power guide. However, given that a whooping five sources over a long period of real-life time assert that Wind Waker!Toon Link is 12 years old, I feel that this is his true age.
I have established that Wind Waker!Toon Link is 12 years old, and the Links in Navi Trackers not only are the same age as him (with the green Link literally being him), but they are also the same age as the Links in Hyrulean Adventure. I now want to make the safe assumption that the manga Links are the same age as the in-game Links in Hyrulean Adventure. I should not have to argue this point. The manga is an adaptation of the game meant to advertise the game. If we make this assumption, this means the manga Links are 12 years old. To me, this is the safest assumption. In the plausible age range of 9-12 years old, 12 years is the oldest, which is certainly more comforting to the reader than if they were younger given all the dangerous situations they go through. The Links also handle their hardships in ways that require more wisdom than I would credit to younger children. The Links are not "mature for their age". They very much act like children. I simply believe their ability to reflect on their experiences and behavior is more plausible if they are older.
With all this in mind, I confidently assert that the Links are 12-year-old children. Certainly, people are free to have their own headcanons, but I believe that, based on the evidence I've seen and connections I've made, that 12 years old is the safest bet.