Awwww, congrats on the new addition! Very adorable, and looks decently healthy! Some of them I see just look so tiny and skinny...poor things...
The babies I usually see APPEAR to be between 5-8 weeks old, and your guy, though hard to say, looks about 7-8. Just like the adult bettas sold at petstores though, unfortunately its next to impossible to tell the exact age with the baby bettas. Often times they are even stunted due to being stuck in those tiny, cold, dirty cups and can take longer then usual to reach adulthood and start to fully color up.
But like I said, you're little one looks pretty good. Just be sure to make sure its water is extra warm(babies do better at slightly warmer temps then adults, like 82-85-ish, so you may need to look into a different heater if that pad one isn't doing the job), keep up with the extra needed water changes, and make sure to offer frequent small meals of a mix of frozen/live foods and crushed high-quality pellets if it'll eat them and hopefully in a few months those little camouflage stripes will disappear and its colors will be beautiful and bright! :)