For a definitive answer, contact your local chamber of commerce or government office. You can also google 'starting a business in texas' and find plenty of resources for information.
But I would say yes, you could still get a permit, for two reasons:
1) The government is savvy enough to know that you are going to sell what you want to sell regardless of having the legal paperwork or not. So they're not likely to make you exempt from the law just because you are 15. If you are selling a product, they want you legit.
2) Sellers permits are typically assigned to business entities, not individuals. So as long as you register your business, you can apply for a sellers permit and I'm not sure your age will matter.
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