As you’re 15 years old, you’re right on cusp of when you’re able to add lean muscle mass. Does this mean you should start training for size by squatting / benching and deadlifting at age 15?
Maybe, maybe not, as there are other things to consider as well.
Bone growth doesn’t stop till about age 20. so you still have some growth yet at age 15. The question is do you want to put these kinds of loads on a skeleton that is not yet mature?
Many medical professionals say no, but many strength trainers ignore this advice and start deadlifting and squatting in their teens. No, these lifts will not stunt your growth but injury is a concern in a skeleton that is still growing.
Another issue is coordination, doing complex exercises like squats and deadlifts require a lot of coordination to perform properly and bad form WILL cause injury. Teens tend to be less coordinated because their bodies are changing size and proportions faster than the brain can figure out how to control everything.
The last issue to consider when deciding whether to start training for mass / size right at age 15 is maturity and judgement. For most teens, the brain tends to lag behind the body in terms of development. There is a reason that the legal age for being an adult is 18, it’s because until that time judgement skills have not fully developed. Before age 18, one is much more likely to do try something dangerous in the gym - such as lifting more weight than you're safely able to.
The split you outlined is fine. Since you’re new to training your body will respond to almost any exercise routine. As you don’t mention the specific exercises you’re doing. I'd stick to basic, body weight focused exercises.
I'd suggest placing your primary focus on developing good form – habits developed now will be tough to change down the road. Consider seeking guidance from an experienced trainer to help with this.
I’d advise waiting till your growth is complete (around age 19-20) and coordination / judgement is under control before doing more complex exercises such as squats and deadlifts.
That being said it's up to you. Do your own (or with a parent) risk / benefit analysis and decide what is right for you.