I’m biased, so naturally I’m going to say that it’s never too late! However, I will be fair to say that the 10 month mark is the best time to introduce signs and at around 14-18 months you’ll receive the fullest benefit to signing, but that doesn’t mean you can’t get great benefits by starting later or earlier.
When signs are taught to toddlers, they can often pick them up instantly. This is great for parents who aren’t as patient. I’ve successfully taught my nephew who was just over 2 years old, 3 signs inside of 10 minutes – to both our delight. Not all toddlers begin speaking right away, so this can be a Godsend. You can teach any sort of sign you want to a toddler, but I would recommend signs that motivate your toddler (words that interest them), as well as signs that make their life easier. These can include signs for things like HELP, UP/DOWN, PLEASE, THANK YOU, SLEEP, FRUSTRATED or any other word they habitually use, but can’t say verbally clearly enough.
When my son turned 20 months, he used the sign for FRUSTRATED often as he struggled with his building blocks and his front-end loader. We taught him the sign after we visibly notice him getting upset by not being able to work the toy how he wanted to. Doing the sign for HELP and FRUSTRATED helped him voice his concerns and virtually eliminate his outbursts by giving him an outlet. Grandma was sure impressed when she witnessed our interaction. This was a clear example of how tantrums are a direct result of not being able to make their thoughts known. While a casual observer might see the interaction as a result of not being able to accomplish a task, the very fact that doing the sign-communicating the discomfort eliminated the outburst. It was the outlet that provided the solution to the tantrum. Doing a sign provides a toddler with an means to get what they want, and while all people must deal with discomfort, not being able to talk about it, makes it far worse.
The FRUSTRATED sign also came in handy during potty training, because our son had difficulty pushing on the toilet. He transferred the sign right away and so we knew when it was time to take him off the potty and let him go in his diaper.
Toddlers also have a very limited spoken vocabulary and since signs can be added almost instantly, you can quickly expand on what a toddler can say. Many toddlers have trouble making certain sounds so signs can be added as replacements for words your toddler finds difficult.
Whenever you introduce signs, just make sure to sign and say the word in tandem and be consistent and sign each time you say it!