Governments set minimum age limits on things for the safety of the individuals themselves and others.
For example, the legal drinking age in Texas is 21 years old. Yet it may come as a surprise to find that the legal age to drive an 18-wheeler is a mere 18 years old. Currently, that applies to driving within the state boundaries, yet recent plans by the federal government aim to expand that to allow 18-year-olds to drive interstate too.
Driving trucks safely requires experience and maturity
When a truck crashes, the consequences can be catastrophic for anyone in close vicinity. Typically that means drivers and passengers of other vehicles, yet it could also mean pedestrians and cyclists.
While an 18-year-old driving a truck hopefully acts more sensibly than one drunk at a party, they can do a lot more harm. However cautious they are, however hard they try to stay safe, young drivers are likely to make mistakes due to their inexperience and the fact that they are so young. Think back to when you were that age. You probably did many things that seemed OK to do at the time, but you would never dream of doing now you are older and wiser.
All drivers make mistakes. You could say it is part of the learning experience. Yet, it makes little sense to allow them to make their mistakes in charge of such a large vehicle that could do so much harm.
If a young driver injures you, consider how much of a role their age and inexperience played. Regardless of what the law allows, trucking companies are not obliged to let kids loose with 18-wheelers. With legal assistance, you can assess your options for holding people responsible, from the driver themselves to the company that employed them.