Answer: If you are talking strictly about beliefs, then I shall explain my answers in the explanation. You may not share my beliefs, but these are my beliefs. The Earth is estimated to be 4.6 billion years old, around the same age as most of the planets in our solar system.
Explanation: Brahmadev, using the power of his mind, created the universe within seconds. Assigning important people to different positions in our solar system and universe, in our solar system, he had assigned already Gods, Chandra and Suryadev (Moon and Sun) to be formed first as a part of the Navagraha. With this, Shiva, the God of Gods, assigned a Goddess, named Prithvi Devi, to be the Mother of Earth, and formed her planet. Alongside, granting positions to Brihaspati (Jupiter) his position when he meditated, Shukracharya (Venus), when he abused his power of a boon, (a gift bestowed by God through meditation) and through the power of his meditation, Mangala (Mars) who through his hard work was recognized as Shiva's son and got the recognition he deserves. He also gave positions to Budh (Mercury) through the power of his meditation, and he gave positions to Shani (Saturn) when he was recognized for the misfortune he had in his life. Alongside this, he also granted Rahu and Ketu to be Gods in the space, when Vishnuji cut off his head due to his betrayal with his Sudarshan Chakra. Now, enough of this unnecessary explanation. I will now explain my answer. Brahmadev has days called "Kalpas" which vary amongst 4-4.6 billion years. So far, in this universe, only one of Brahma's Kalpas has passed, and during these Kalpas, are Manvantaras (330 million years), the fourteen first humans called Manus change. The current seventh Manu is Vaivasvata, and the current King of Demigods called Indra (position) is called Purandara or Shakra (person). During these Kalpas, Brahmadev is said to be in constant meditation. Only after each Kalpa passes, for one Manvantara only, he is awoken from his meditation. So far, this is the 14th Manvantara. This is the current estimation about the life of the universe till now. As described in the Narada Purana, there are six more of these Manvantaras before the universe is destroyed.