Old enough to say “I’m asexual”.
There’s no minimum age for asexuality, just like there’s no minimum age for any other sexual orientation. You never hear anyone say “Well, you’re only 15, so just to be safe, you might want to give it a few more years to see before you rule out bisexuality. You never know when some hot guy might catch your eye!” That would be ridiculous.
Presumably, by the time someone is at the point where they’re comfortable with identifying as asexual, they’ve spent some time thinking about it. They’ve gone through the process of realizing that they’re different from their friends and wondering why they’re not as interested in sex as everyone else around them. They’ve spent long hard hours questioning themselves, trying to figure out who they are.