Teenagers have always had one of the highest suicide rates anyways... Blaming it on parental gender reinforcement won't change that. But the child needs the maturity to understand why they feel the way that they do.
Abuse is probably the prevalent underlying cause of gender confusion.
I think you misunderstood me somewhat, yes teenagers have a high suicide rate, but transgender and non-binary teenagers significantly more so than their cis gender / binary peers.
Eg. an anonymised study by the American Society of Paediatrics:
More than half of transgender male teens who participated in the survey reported attempting suicide in their lifetime, while 29.9 percent of transgender female teens said they attempted suicide. Among non-binary youth, 41.8 percent of respondents stated that they had attempted suicide at some point in their lives.
Doubtless you could dispute the tenacity of their suicide attempts, but arguing that "when you're older you'll know better or feel differently" which I'll wager is a common parental stock response, neatly tagged on to "its just a phase" is pretty lazy thinking and doesn't help the individuals in question.
Also, implicit in the argument that people should wait is the the idea that supporting them to live as the gender with which they identify is in some way harmful, either to them or to society, which I would dispute.