Quick fun fact I learned in ASL class:
Babies can begin signing as early as a few months old. You don’t have to wait until they’re 9-12 months to start communicating verbally; the parts of the brain that process and use language develop before a baby is able to speak intelligibly with their mouth. Teaching your kid sign language early means that they can communicate effectively months ahead of schedule, when compared to peers that only speak a spoken language.
Additional fun fact: this jumpstart in language is thought to be a possible way to avoid the “Terrible Twos”; that phase of a toddler’s life is thought to be largely due to a toddler being unable to effectively communicate their needs. If a two year old has already been speaking for a year and a half, they’re far more able to communicate to you what’s wrong. Heck, they might also start reading earlier; languages with a fingerspelling component, like ASL, mean that any speaker needs to be able to spell unfamiliar words and ask about them. This can jumpstart a toddler’s ability to recognize letters as components of a word, and teach them to spell, read, and eventually write these letters to communicate.
Which, of course, lends absolutely zero credence to the theory that ASL will inherently stunt someone’s spoken language skills. If anything, sign language fluency makes acquiring any language, spoken or not, easier rather than harder.