In reply to Monk:
Climbing at your limit and not being able to climb because you are a beginner and haven't got a clue how to climb properly are subtly different things.
The sensations of achievement you get whilst attempting to mastering a new skill are very different from the kinesthetic and mental thrills from proficient execution of it. That is true of almost all sports but in no way denigrates the effort needed to attain the level of unconscious competence in any discipline. [You have four levels of progression in most disciplines - unconscious incompetence, conscious incompetence, conscious competence and unconscious competence.]
The comparison is only valid if you are comparing like with like - i.e. climbers who are unconsciously competent in their general approach to climbing. If they are not yet at that stage then 'physical training' in the context of this thread is not applicable as are the associated comments that we are debating. I certainly don't know any such climbers who fall off 5s. However, I certainly know many such climbers in less than great states of physical health onsighting in the mid-6s.
I'm more than happy to discuss 'where proper sport climbing' starts - you are correct about the fact the grades I quote are rather arbitrary and it will vary person to person [IIRC Steve McClure's first redpoint was 8b]. I was talking primarily about redpoint grades and I think your suggestion of 6b+ for some people is a rather low bar for any motivated individual of average fitness, even one who has only climbed perhaps once a week indoors for a year. For a regular sport climber with motivation, who is neither unwell/injured nor over-weight, working 7a is very, very achievable. What I will give you is that surprisingly few UK climbers seem to fully embrace 'sport climbing' early in their climbing careers which greatly distorts perception of what is or is not easily achievable - perhaps I underestimate the difficulty of redpointing 7a+/7b but I certainly known that many people over estimate the difficulty.