you are so young that you can‘t start doing things.
you are not old enough to start doing things.you are so young that you can‘t start doing things第二个you可以去掉么？原句没有也不行so that最大的区别就是too to 后面加短语，而so that后面加从句~所以从句要有主语么？从句是一个完整句子，必须要有主谓宾~