Namor had to be older than he was, who was tapped as a mutant in the first year.
(When did they decide that Toro was a mutant?)
Chuck said that he was the first mutant because...
1. It helped him pick up chicks.
2. The death Squads wouldn't be looking for all the other mutants if he insisted that his lot were the first five, but that's like presupposing hopelessly that his lot were going to be the new Jews.
3. Chuck had government ties in the beginning. Strong canonical ones. If he admitted that there were hundreds of thousands of mutants, and thousands more being born every day... Tricky Dick was a hair from the Whitehouse. They would make lists, even with the best of intentions, those lists could wind up in anyone's hands.
4. Charles could have meant that he was the first Omega Class Mutant? First tier, or most powerful, or even a political rank of "First" like "Prime".
5. He could have been compartmentalizing his stories to avoid being mind read by his opponents, and even allies, I mean Charlie assumes he's the worlds most powerful telepath, but at that point, according to Stan, it's unlikely that he'd been in any mind wars... Meanwhile according to Chris years later Shadow King completely owned him in thier first encounter when Storm was still prepubescent. Vain, pompous and delusional.