According to the evolutionary theory, comets are supposed to be the same age as the solar system, which is said to be about 5 billion years old. But each time a comet orbits close to the sun, it loses so mush of its material that it could not survive much longer 100,000 years.
1) Evolution deals with the origin of species, not comets.
2) The amount of time a comet survives depends on two things, a) the mass and composition of the comet b) the orbit of the comet. If we take Hale-Bopp which has an orbit in the region of 4000 years it's probably going to last longer than a comet like 73P/Schwassmann-Wachmann 3 which has an orbit of about 5.5 years, in fact it does as 73P is breaking up before our eyes. You can see my images of these two comets on my gallery. To claim a single number for all comets is silly, some comets have an orbit of 37 000 years, there are probably some comets out there that have orbits of over 100 000 years.
a. Comets come from an unobserved spherical "ort cloud" well beyond the orbit of Pluto.
No one has ever seen this so called "ort cloud" and there is simply no way to prove that this exists.
Ohh I can't resist this one... No one has ever seen this so called "god" and there is simply no way to prove that this exists. Oh that felt good.
We have seen dozens of parts of the post-Neptune rubble field (we call it the Kuiper Belt), which was detected in 1992. With several major discoveries in the last few years, the extra moons of Pluto, 2003 UB313, Xena and so on. The extent of material out from the Sun is up for some debates, some comets have extremely long orbits which would imply that it extends for quite a distance.
b. Improbable gravitational interactions with infrequently passing stars often knock comments into the solar system.
Nobody claims that in the manner you imply. Material in the Kuiper Belt and beyond will be slowed down by the passing of Neptune and other bodies, yes even other stars, they'll lose energy and plunge inwards, impacts are another reason, even objects outside the solar system could be captured by the Sun. All the left-overs of the birth of our system won't just vanish.
If evolution is true then how come we still have comets?
Because evolution deals with the origin of species not comets. The fact of the matter is we don't have many comets anymore, it would of been much better to be an astronomer a couple of billion years ago.