The idea that the planets formed at very different times, so that e.g Mars was presumed to be older than the Earth and Venus younger (or vice-versa), goes back to the beginning of the nebular theory, well over 200 years ago. Nowadays no planetary astronomer (many of whom prefer to be called planetary geologists) takes such simplistic theories seriously. Instead, theories tend to be based on other considerations, as discussed in the following paragraph:

The difference in the composition of the planets is not related to when they were formed, but where. Close to the Sun it was too hot for ices to form, so only small rocky bodies could form. Much further out, it was cold enough for ices to form, and since the elements required to make ices are far more abundant than the ones required to make rocks, small dirty snowballs would have formed, but as gas tried to spiral toward the Sun, the closer you get to the Sun and yet still have ices, you end up with much bigger dirty snowballs, which can use their gravity to hold onto the far more abundant gases (primarily hydrogen and helium and compounds involving hydrogen). That suggests that the forerunners of the rocky planets and the dirty snowballs and the cores of the gas giants were all forming at about the same time. It is possible that on a timescale of a few million years, one or another of the planets might have formed first, but collisions between proto-planetary bodies probably took at least a few tens of millions of years to more or less fully form any given planet, and other than things like asteroids and comets, all traces of the events going on during that time have been erased by collisions and geological events (for small bodies) or are buried deep within the gas giants. So all that can be said with any certainty is that they were all more or less the size they are now within 100 million years or so, and since there has been about 4500 million years since then, for all practical purposes they all formed at the same time.

Dear Sunil Kumar Viswambharan Pillai,

SKVP: ... the first formed planet after sun should have most dense planet. Its our earth ...

Earth was the last to form. There was originally another small planetesimal which collided with Earth some time after they had all formed. The two dense molten cores merged into one but some of the less dense mantle was sprayed into a disc around the planet. That later coalesced into the Moon so it is less dens than the planets wile Earth is more dense.

SKVP: The Last formed planet should be Saturn, that is why it has ring ...

The rings don't last very long. They are probably formed when a comet is gravitationally captured and gets broken up by tidal effects. Rings may have been formed and dissipated many times over since the planet formed. The same applies to Neptune's rings.

The Earth isn't densest because it's made of considerably denser stuff. It's dense because a collision with another (proto)planet made it the most massive of the rocky bodies, and as a result, the one with the strongest compression of its inner regions by its gravitational force. Take away some of its mass and relieve that compression, and it wouldn't be as dense as it is. (The two proto-planets involved are generally called Gaia -- the bigger of the two -- and Theia -- the smaller of the two; and debris from their collision and merger is almost certainly what caused the formation of the Moon. All of which presumably happened between 20 and 50 million years after the start of the condensation of interstellar gas and dust that ended up as the Sun and planets.)

Courtney Seligman, the collision model is the best fit but the nickel-iron cores of both protoplanets remained in the Earth while the less dense crustal material was shared between the Earth and the Mon., that's why Earth has higher than average density while the Moon is lower than average. Compression is also a factor as you note but it is not the whole story.

George Dishman You are correct in stating that the cores of the two proto planets are now in the Earth, and only mantle materials are in the Moon; That's one reason that the Moon is considerably less dense than the Earth. But I was thinking in terms of our core compared to the core of Venus, which is the closest planetary comparison to the Earth, so I didn't consider the Moon, or for that matter, any of the other moons in the Solar System. Sorry if that made my comment less clear than it should have been. (Note: "The Structure of the Terrestrial Planets" at http://cseligman.com/text/planets/innerstructure.htm has a detailed discussion of the structure of the Earth and a comparison to the other terrestrial planets, and the final illustration shows that other than knowing that the Earth has a solid core, the structure of Venus is very similar to that of the Earth. "The Internal Pressures of the Planets" at http://cseligman.com/text/planets/internalpressure.htm discusses (with some illustrations) how the difference in internal structure and composition affects the gravitational force at various places inside the planet, Since I retired about 10 years ago I never got around to updating the pages to add further interpretations of what is already there, but anyone using this site should be able to infer the results on their own.)