Training legs is good at any age, just don’t go stupid with it (or with any other body parts) and eat sufficiently. It’s actually relatively hard to go heavy enough on legs for it to be a skeletal risk (compared to upper body), because you’ve got so many points of failure: technique, core stability, trunk strength, cardiovascular fitness, actual leg strength/endurance. This is especially true when you’re doing sets of 10+ reps, which, as a 14yo, would be wise (the only reason to do fewer reps at your age is because of your technique failing early in the set or doing certain exercises that require low rep ranges to be useful).

I’m going to link you to a program I put together at the start of the year, which is really, really, really simple. The program, as written, is intended for adults, so I’ll make one amendment to it for you as a teenager: always stick to a 7-8/10 RPE. I’ll explain what that means in a moment, but first, here’s the program: One More Plate: A Basic Novice Program for Strength and Muscle

Okay, so RPE means Rating of Perceived Exertion. In strength training, an RPE of 10/10 means that the last rep you did was the most you possibly could have done: you had nothing left over. 9/10 means it was hard, but at the end of the set, you could have done 1 more rep. 8/10 means you could have done 2-3 more reps. 7/10 means that the last rep you did could have been really fast on the way up if you wanted it to. The two things that are most likely to cause skeletal issues in teens from weight lifting (other than bad technique or general injuries) are going too heavy and not eating enough. Sticking to a 7-8/10 RPE will protect you from going too heavy. Once your bones are finished growing, then you can start working at 9-10/10 RPE if you so desire.

The other piece of the puzzle is to eat sufficiently. How much food is sufficient will vary from person to person, but given your height and weight, I’m going to suggest eating a lot of wholesome food (vegetables, meat, fruit, dairy, eggs, nuts, legumes, whole grains) and to overall increase your serving sizes by about 10% from what they currently are. In time, you might increase serving sizes again, but this is where I’d probably start.