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The reference to age limits in humans was simply an analogy about limits per se.

The issue with speciation is whether it reaches a limit or does not. Is speciation an example of how maximizing certain pre-existing traits can lead to loss of others? Or is it a stage in the evolution of one thing into another?

That's the debate. The evolutionist would propose that random mutations over time account for the diversity of life on earth. That's an awful lot of diversity. Millions of species. Some fly. Some swim. Some are huge. Others are so small they can only be seen through a microscope. Some split to reproduce. Others have complicated courtship rituals, followed by mating, gestation, birth, nurturance, etc..

I don't object to faith at all. I have it myself. Perhaps I've inadvertently left the impression that I devalue faith by stating that belief in evolution requires faith (and a lot of it) but that was not my intent.

On another issue you raised, I understand that deliberate breeding produces results different from the more random variations we see in nature. It's doubtful nature would produce a chihuahua. But the lesson there is that deliberate efforts to stretch natural boundaries eventually reach a limit. We can't breed a dog the size of a flea or the size of an elephant. We can't produce anything from breeding fruit flies other than fruit flies. What would lead anyone to assume that random mutations would ever lead to anything radically different from what was there to begin with?

How long would we have to observe micro-organisms dividing until somehow we ended up, through random mutations (and it would surely take a ton of them), with a larger creature that reproduces sexually?

Might it simply be that such a thing never happened? That giraffes have always been giraffes? That the variation we see is in fact limited. Natural selection takes it to one level, and we can push it to another through selective breeding, and then CRASH! We hit a wall that neither we nor nature can breach. You simply CAN'T produce dogs the size of an elephant, and neither can natural selection and mutation.

Nor can natural selection and mutation lead to micro-organisms evolving over eons into the millions of life forms we see today.

If you believe in evolution, you must believe that the limits we have seen in everything from amoeba to fruit flies to dogs can be breached and have been breached countless millions of times by random chance. Maybe that's true, but it's speculation. I would suggest that those limits are unbreachable by any natural process. The number of mutations necessary to account for all the life we see would be unfathomable.
Anyone is free to suggest that such an incredible, and unobserved, series of mutations happened. Perhaps it did. So offer it as a theory. But don't shout down other ideas, or use the circular reasoning we so often see (e.g., those countless millions of mutations had to occur because without it there would be no evolution). That's like the atheist who once said, "spontaneous generation of life seems impossible, but it must have happened because here I am!"