I don't think you have only one soulmate. I think there are multiple people out there who are just right for you, so I guess I think you have a lot of soulmates to choose from.
I think you can meet your soulmate at a young age and know you've found the person you want to spend the rest of your life with. However, I do think love is hard work. You will disagree. There will be times when things are rough. I think you know it's true love when you get through those things together. Your soulmate, imo, should be someone who inspires you to try and be the best version of yourself even when life is crap and you really don't want to be.
I'm 24, and I feel like I've found my soulmate. I've known her since 2014, but we got together at the start of this year, and, like, I just know. She's my best friend, and I can't imagine my world without her in it. This is the first relationship I've genuinely felt like this about. Her needs and aspirations feel just as important to me as my own.
OP, you very well might have found your soulmate, or you might not have. My advice is don't think too much about it. If you're happy, then go for it. Just because you're young and might break up in the future, it doesn't make your feelings right now any less valid. Do be wary of a 20 year old dating a 16 year old though. At 20, I would have never dreamed of dating someone who's only just finished their GCSEs.