Those are ages for puberty NOW, not historically.
This article provides some overview - highlights include German researchers providing statistics that around 1860 the average age of puberty onset in girls was 16 years, not 11. In other words, in prior centuries the norm was to start puberty at an age where kids these days are finished and done with the process. Our “normal” is not, in fact, normal when viewed historically.
I don’t have time right now to dig up a primary-source research paper on the topic, but a quick google will show you ample information that the average age of puberty dropped by 5 years in the 20th Century and that puberty in the age range of 15-17 was, historically, the norm in many places and eras.
Another problem with tracking the long term effects of puberty blockers is that their use in young transgender people - that is, presumably people who are biologically normal for their birth gender - is relatively new. Other uses involve kids with precocious puberty, which untreated can have its own problems (such as increased risk of cancer over a lifetime), and a subset of kids with things like hormone-secreting tumors who, again, have medical issues that may or may not affect long term outcomes.
So yes, there probably are potentially bad side effects. The same can be said for untreated gender dysphoria. That’s why professionals need to be involved in evaluation of the problem and treatment decisions. This is not something DIY.
But also consider that no one is going to be giving puberty blockers to a 3 year old. The only way a kid, say, 8 years old is going to be giving puberty blockers is if they are, in fact, in precocious puberty for which puberty blockers are a long-standing treatment. A transgender kid may be on the blockers fewer years on average than kids with precocious puberty would be, although exact lengths of time depends on the individual.
The fact is we routinely give kids, even young kids, powerful medications for various reasons, all of which can have side effects both short and long term. While certainly puberty-blockers shouldn’t be handed out lightly in conjunction with on-going treatment for a problem I don’t see where they should be ruled out or viewed as something totally new when, in fact, they aren’t. This particular use is new, but the medications are not.