Can you have a gun in your car if you are 18 years or older?

It depends.

First, the person possessing the gun cannot be a convicted felon, or if he/she is then their 2nd Amendment right to bear arms must be restored. If the person has been convicted of a domestic violence offense (even if it is a misdemeanor) under Federal law they cannot possess a gun unless the court has set aside the judgment of guilt.

Second, the gun cannot be concealed in any way unless the person has a concealed weapons permit. Otherwise it is against the law to conceal a weapon in the car. Some employers do not permit their employees to bring guns to work even if they are left in their cars. Assuming no convictions for anything and the person is over 18, yes it is ok if it is not concealed, but no if it is concealed.