The question of whether you need a toddler bed rail, depends largely on how active your little one is. But the question of when to use the bed rail, is a decision for Mom and Dad, and one that should not be taken until the child is at least two years old.
Some children are more precocious than others, and even with their crib mattress in the lowest position, they can be scaling the heights and heading over the rails before you know it. Most toddler beds are not designed to use rails with. Generally, they are low enough that falling out of bed is not a serious issue. And at this age, a toddler who wants out of the bed, is going to get out, regardless of what measures you take.
Twin Bed Rail
A toddler benefits most from a bed rail, when they reach the size that a regular twin size bed is needed. These are the ones with more height, when you combine their box spring and mattress. That means a longer distance to fall.
Most models of toddler bed rails are made to fit between a mattress and box spring, using some variation of an L-shape. The actual height of the rail varies, and can be affected by the depth of your mattress. This is an important item to consider when looking at the size of a rail. If you are left with only a few inches of protection, a restless child can roll right over the top at night.
Another consideration, is the construction. Toddlers can be restless, and temperamental. Banging their heads on rails can cause bruises and a bigger tantrum than they were having to start with. You may wish to consider rails that are padded, for extra protection.
Before looking for a bed rail, measure the bed's height, and the mattress depth. Then compare the features offered by each manufacturer of rails, including padding, portability, and ease of use.