The risks aren’t of taking blockers, the risks are the effects of not having hormones. Hormones are essential for many aspects of health. To leave someone without hormones for a prolonged period will cause issues.
For comparison, if a cisgender child has not started puberty by the age of 14, we start to worry. There’s no immediate hurry, but just remember that hormones are needed.
More info: Puberty Blockers for Transgender Youth and Bone Health, Do I need bone scans or an X-ray before hormone treatment?