You have to apply for it by the time your child is 18 or before you wish to travel to the US. As your child is a US citizen, they have to enter the US on a US passport.
However, it is not a taxing/complicated/stressful process. You just need to gather the required documentation that proves you are eligible to pass on your US citizenship (I used my high school and college transcripts), a few other documents (child's birth certificate, my US passport, wife's UK passport, and our marriage certificate), went to the embassy appointment, handed over my documents and all of the application forms (crba,passport, and SSN) paid the fees, had a coffee and then my wife and I took an oath for the little man. 3 weeks later, CRBA and passport in hand, and about 10 weeks later his SSN.
Embassy staff were really nice, and even renewed my US passport while I was there. Once you have all your documents, join the telegram group (there is a link somewhere) to be notified when appointments become available as they are randomly released.