Summary: there's no minimum age; rather, you must use the seat in accordance with the manufacturer's instructions. Instructions for reversible seats typically do not depend on age but on the child's weight and height.
Arkansas law (AR Code § 27-34-104 (2020)):
(a) While operating a motor vehicle on a public road, street, or highway of this state, a driver who transports a child under fifteen (15) years of age in a passenger automobile, van, or pickup truck, other than one operated for hire, shall provide for the protection of the child by properly placing, maintaining, and securing the child in a child passenger restraint system properly secured to the vehicle and meeting applicable federal motor vehicle safety standards in effect on January 1, 1995.
(b) A child who is less than six (6) years of age and who weighs less than sixty pounds (60 lbs.) shall be restrained in a child passenger safety seat properly secured to the vehicle.
(c) If a child is at least six (6) years of age or at least sixty pounds (60 lbs.) in weight, a safety belt properly secured to the vehicle shall be sufficient to meet the requirements of this section.
The key passage is "properly placing, maintaining, and securing the child in a child passenger restraint system properly secured to the vehicle." This implies that you can put the seat in the forward-facing position only if it is designed to be used that way and then only when the manufacturer's instructions say that it's acceptable. In my experience this typically depends on the child's height and weight rather than on the age. For example, one manufacturer says of its seats
[Convertible seats] in the rear-facing mode should be used for children weighing 5-40 lbs, with a height of less than 49 in. and whose head is 1 in. or more below the top of the car seat.
[Convertible seats] in the forward-facing mode should be used for children weighing 22-65 lbs. and height less than 49 in. and can walk unassisted and whose top of the ears are below the top of the car seat.
So if the instructions for your seat have the same criteria then you can turn it around when the child reaches 22 lbs. CDC weight data for boys shows that virtually every 21-month-old boy is heavier than this (note that the data are in kilograms and that 22 lbs is 9.979 kg).
Therefore, the legal answer is that there is no minimum age; the requirement is to use the device "properly." Follow the manufacturer's instructions.
(Aside: the incorrect belief about the 2-year threshold could be a widespread misconception among Arkansas police. If this is so, and if they are therefore in the habit of ticketing people for having children under 2 in a forward-facing seat, then you could get a ticket. You might be able to convince the officer of the reasoning in this answer, but you might not, in which case you would have to take it to court to show that you were following the manufacturer's instructions and therefore the law. You will have to weigh the potential hassle of having to do this, including the likelihood of its happening or not, against the benefit of letting your child see the back of your head while you're driving. My guess is that you're unlikely to get a ticket for several reasons, and that your child is likely to be significantly happier facing forward, but I don't know Arkansas police and I don't know your child.)
If you do not have the instructions for your seat, you should be able to find them online. Some seats are not designed to be used facing forward. If yours is such a seat, you will need to get one that is.
Remember that complying with the law may seem more important because you're more likely to get pulled over by a police officer than to be in a collision. But in fact the most important consideration is safety. If you are in a collision you will want your child to have the greatest protection possible.