To both the young dudes.
There is a lot to be said for weight trianing in youth and even my son is commenting on a hint of lines being the start of his six pack due to training. He is younger than either of you and a long way from having defined abs, fortunately.
My son is at an age where this is just a notion and isn't feeling any peer pressure about it, just heard the term and knows he's getting fitter and stronger. In a few years he will be more like you two and start wanting to look like people in films and magazines to impress the girls and I will have to make myself really unpopular with the person I care for most by saying the same to him as I am saying to you now. I have copied it form a post I made to someone wanting aesthetics at roughly your age recently, with little modifications.
In the process of preparing for my home educated son's PHSCE I have recently had the joy of studying puberty, so found out how easily I could advise you really badly.
Your age makes it really difficult to advise how high or low your body fat level should be as dependant on the stage you are at. You could be coming up to a stage where you will lose a lot of this naturally and having too little could delay your developement. I also know that you are 99% unlikely to know this yourself as well so the below is uber-cautious and giving a serious warning not to aim for something insane like a six pack until you have definately finished pubescence, sorry if this is not what you want to hear, but it is truth.
Restriction based on age alone regardless of puberty. Your bones will most likely still be growning or have finished but not fused. As such extreme low rep work is off limits. The lowest you should ever go to is 10 reps on any exercise and this should be at a weight where you feel you could do at least 2 more at the end.
I am going to assume you have not done weight training before and have either no or very little equipment. The below is a simple balanced circuit I have suggested many times before using the BALS system for balance (Back Arms Legs Stomach) use a loaded rucksack for weight where there is an * if you dont have any weights. None of this should be to the point of failure for you currently, but doing it now will mean you will have strength and endurance enough to do so as soon as you are old enough.
If you don't know how these should be performed please google the name for demonstrations.
No rest circuit either timed at 1 minute per exercise or 15 reps of each. Start off doing 3 circuits and if this is too easy add more.
Back - Bent over rowing *
Arms - Push ups, can be box style, straight body etc. based on ability
Legs - Squats *, depths is a point of contention for many, do what is comfortable and if you hear clicks it's too far. You'll click and creak enough in years to come don't worry.
Stomach - Crunches
Warm up before your workout by cycling, jogging etc. for 5 - 10 minutes.
Use cool down a chance to do some longer cardio, cycling running etc. for 20 minutes and do a well rounded stretch out for all of your body, these are easily found on line, static stretches only no bouncing into movement or targeted movements.
Doing this should be safe, it will build you muscluar strength and endurance. It is not inteneded to last more than a few months, as most sessions aren't. If you enjoy it come back for more in a few months, if you hate it tell us and I or someone else will give a better alternative, as long as you say what was so dreadful.
Justins body weight exercises are a good place to look for variety as well.
