Hi. I'm qualified to level 2 in British Sign Language and my mum has her own voluntary group where she teaches sign to any age (for free) so there was ample opportunity for my LO to be taught if I'd wished. However, I chose not to, I have my reasons and opinions for this choice and like anything it is down to personal choice and how you choose to spend time interacting with your child. My choice was to limit sign use to 'food' 'drink' and 'toilet'. (I repeatedly asked my mum to stop signing in front of him, I chose that I didn't want him spending his time and effort learning hundreds of different signs which he could have been exposed to)
It's never too late, a child of any age, including a toddler or an older child, a teen, an adult and the elderly (often with hearing loss) can learn to sign and it's a very useful form of communication. Any time you spend with your child interacting is worth while whether it is sign, speaking, playing with toys, reading books, whatever, so in that sense it's great, it can also help to ease the frustration little ones have in communicating their needs when they are pre-verbal. That's why I chose the few signs I did teach, to reduce frustration and help him get what he needed by being understood quickly but from minimal effort being focused on acquiring a second language. Food, drink and toilet are simple signs and in fact you don't have to use official signs you can use anything you like, other carers are likely not to mind being asked to learn just 3 (or half a dozen) signs if they are simple to pick up, it's not the same as thrusting a book upon them.
Just briefly I'll explain that I chose 'food' rather than hundreds of different food items because once LO communicates they are hungry it is easy to run through a list (or point to things) and for them to indicate what they want where as if they know the sign for banana but really want a piece of toast they are stuck asking for banana. My LO signed almost identically for food and drink and it really didn't matter, he had made his need known well enough.
I honestly do not think that teaching sign will help his verbal language skills develop any quicker though so if that is your motivation then I would probably suggest just talking, reading books, looking at pictures, describing everything around you and everything you are doing (I'm sure you do this already). At 18 months he is on the cusp of some huge language leaps. Between now and 2yo (or a bit older) he is going to have language explosions that will blow you away. Although he has some words now language doesn't really come in one or two words it kind of appears seemingly over night, there is likely to be some terrible sleep disruption and at the end of it your LO will suddenly know dozens more words or start chatting. For instance at 17, 18, 19 and even 20 months I could write down the words or sounds my LO was able to say, at 21 months it was pointless trying to record what he could say, 4 word sentences, the entire alphabet and counting from 10 backwards down to zero.
It's a very exciting time and I think you are almost upon it!
Hope this helps some in making your decision.
