You can tell the trustee how to invest, when to withdraw. It's your money it's up to you to put in whatever you want.
However, I am not sure an 18-25 year old is mature enough to follow the trust condition. For example, a few years ago if someone got dragged into bitcoin, it might be tempting to move some money out of trust to invest in bitcoin because they believe firmly they will make 10 times in return. I can see a 23 year old smartass kid do that in violation of trust requirement.
This is why the trust is usulaly managed by a trust lawyer or company for young adult beneficiaries.