This argument is usually only used for people who don’t identify as cis or straight. You’re assumed to be cis and straight. Those are the factory settings. Saying, or rather confirming that you are really cis and straight, is not necessary. We already know. You’re “normal”.
If you know that you’re different, people often question your capability or your maturity to make that assessment. But why? If someone tells you what their favorite color is, you don’t tell them that they are too young to know. You accept their answer. So why question their sexual orientation/gender?
I understand it’s a little bit different to identify with not feeling sexual attraction toward people than it is to do, but then again most sexual orientations actually include the not factor.
Let’s say you identify as straight. That doesn’t just mean you do experience sexual attraction towards members of another gender, it also means that you don’t experience sexual attraction towards people of the same gender as you.
Does that mean you’re too young to know? Maybe when you’re older you’ll start to experience sexual attraction towards members of your gender? It’s possible, but not something worth holding your breath for.
Same goes for asexual people.
People discover their identities at different times in their lives. Some people know it sooner, others need longer to figure themselves out.
When someone tells you they are ace (or anything else), believe them. Or at the very least, don’t patronize them.