Why is Pluto no longer considered a planet?

 The reason that Pluto is no longer a planet is not because of its size. In fact, it passes the test for "size" (mass).

The definition of 'planet' was made more strict. It is as following:

1. Massive enough to be round. Very massive bodies have so much gravity that they crush down any irregular edges towards their centre, and so become ball-like.

2. The primary object orbiting the Sun. For instance, the Moon orbit's the Sun, but it does so by orbiting the Earth. The Earth is the primary object orbiting the Sun.

3. Has cleared it's own orbit. Planets clear their orbits of debris/asteroids (by attracting them with their gravity).

Pluto passes 1 and 2, but has not passed 3. It has not cleared it's orbit of debris.

Objects like Pluto are called Dwarf Planets.

Are all the planets in the solar system formed (almost) at the same period?

 The gas giants are first generation planets and were formed 4.6 billion years ago: first Jupiter and Saturn, then Neptune and Uranus.

Jupiter is said to be the oldest planet in the solar system and was formed shortly after the Sun. It would have been followed by Saturn.

We know this because unlike Uranus, Neptune and the rocky planets, Jupiter and Saturn are rich in helium and hydrogen.

By the time the other planets were formed, much of the hydrogen and helium in the primordial cloud in the solar system had disappeared. Jupiter and Saturn are representative samples of this cloud.

Among the rocky planets there are first, second and perhaps third generation planets.

According to the "giant impact hypothesis", the Earth is a second generation planet formed by a collision between rocky planets of the primitive solar system, Gaia and Theia.

These two planets collided 4.468 billion years ago, more than 100 million years after their formation.

The hypothesis proposes that Theia was formed at a Lagrange point of the Gaia-Sun system, at the same distance from the Sun as Gaia, but forming with them an equilateral triangle.

After Theia had reached the current size of Mars, 6,500 km in diameter, the other planets would have destabilized it and caused its collision with Gaia.

Theia would have struck Gaia at a speed of 40,000 kilometers per hour at an oblique angle, ejecting portion of both Theia and Gaia into space and creating a ring of debris around the new Earth. By accretion, between one and a hundred years later, half of this debris would have given birth to the Moon.

Mercury would also be a second generation rocky planet. Indeed the core of Mercury has a higher iron content than that of any other planet.

Mercury would have been formed by collision between an ancient planet of approximately 2.25 the current mass of Mercury and a planet of approximately 1/6 of this mass. The impact would have removed much of the mantle of the old planet, leaving the core behind at the base of the formation of Mercury.