Unless she got her own tattoo. Could be why it's that odd bright green colour.
I think what Gervaise said is more likely and Dalish's clan just kicked out the mage that was oldest and had the best shot at making a go of it. Dalish could have been anywhere from mid-to-late teens onward and gotten her vallaslin. Heck, it might have been a parting gift. Make her an official adult before she goes kinda thing.
@Gervaise: As for the illogicalness of the 3 mage rule, I can see the point. However, consider the following:
Regarding the idea that, "if they're that afraid of abominations, then why keep any mages at all?". The simple answer to that is tradition and the Dalish desire to cling to whatever tradition they still have. The 3 mage rule could be seen as a balance btwn the desire to keep to the old way of having a mage run the group (which I personally think might be a hold over from all the way back in pre-veil Arlathan) and the danger of a clan being overwhelmed if a mage goes abomination.
You have a Keeper and an heir (the First) that that the Keeper can devote all their time and spare attention to teaching, decreasing the potential for (aggressive) possession as much as possible.
You add a spare heir (the Second) to that and you stretch your resources a bit. You also have a mage who's possibly gotten less personal attention and training from the master, and who may also have more cause to feel jealousy and other negative demon-attracting emotions towards the more favored First (for example: as Lanaya basically says the dalish mages she beat out for the position of First felt toward her). There's more chance of a bad situation resulting, though its still manageable. And its necessary to accept the risk, in case the spare is actually needed.
Now, a third mage is even more of a risk with less justification to take that risk. They have the increased strain on resources and resulting possibility to fall into possession due to lack of preparedness, plus the possible Bhelen-like feelings of the thirdborn who feels they should be the next to be in charge. And yet they have none of the benefit to the chain of succession the Second does, only being truly needed if somehow the First and Second both get taken out of the picture. This leaves the clan with the incentive to either pass off a third (and definitely a fourth or fifth) mage to another clan who is lacking in mages, or to turn out one or more of the mages in the clan.
Now, its true that even one mage could become an abomination. But the three mage rule seems to be an attempt to strike a balance between the desire to keep the Keeper tradition alive and the realities of the Dalish system of Keeper succession and the scarcity of their resources (like being unable to group together with other clans and have some kind of school system that could free up the Keepers from the responsibility of typically being the sole teachers of magic for their clan's mages while also acting as chieftain of the clan as a whole). Yes, one or two mages per clan is still a risk, but its one they're willing to accept b/c the return is worth it. Three or more seems to be the point of diminishing returns.
True, Zathrian's clan still has more than three mages, so it's still a valid example.
It does? We have Lanaya. Zathrien, himself. Who else am I forgetting?