Each group has its conditions that you have to review. South Asians have a history and social structure even today that is extremely stratified. That stuff is conducive to differentiation in many ways.
From today's perspective, our cultural definition would not be as it was before Somalis were Muslims. The conditions changed. Af Maxaa is the latest configuration. And by the way, linguistically Maxaa-Maay might go back to 2000 years since proto-dialectical-continuum can be a long process before you see divergence because of changing conditions.
And there is a flaw here as well. You want to point to the time when you could define Somalis as you largely do now, right? That's not reasonable because go back to that time and you would have struggled to put an arbitrary cut-off point since we are not a group that diverged from a different ethnic group which is usually the case, we're basically the same group that changed over time. That is why I go back to when we can conclusively say it is roughly the source point. The actual roots where even if things were nascent and interesting, most of that would evolve into what we are today. That is why it is our ethnogenesis.
You say Islam, I say, indeed. I agree. That is what we are today, but that is the latest groundbase step to a process of people that goes way beyond that in time. Do you understand? Basically, we're the same people with an old continuity. Ethnically the conditions have changed, but ethnically it is based on a longer thread tied to a continual group that has not received any meaningful population turnover.
The question of the thread was how old is the Somali ethnicity. I think I roughly answered that.
Essentially, the question of what is a Somali today is not the same as how old is the Somali ethnicity. These are very different things.
Furthermore, the word ethnicity is tricky since it cannot serve enough justice.