Athena's advice to Telemachus can be found in Book 1, lines 296-97. She is telling him that the time has come to leave his childhood behind and be a man, saying, “…You should not go on clinging to your childhood. You are no longer of an age to do that."
Dr. Walter Englert, of Reed College, explains the advice very well. He says, "These words could be a direct quotation of a mother or father dropping their hesitant son or daughter off at college, and with them Athena is giving Telemachus needed advice. Telemachus is in a tough position as the Odyssey opens, and he needs to grow up. When Athena speaks these words to Telemachus, his father Odysseus has been away for 19 years. He left when Telemachus was just a baby, and Telemachus has no memories of his father. For the last three years, the situation in their house has been desperate: 108 suitors, convinced that his father Odysseus will not return and attracted by the beauty and intelligence of his mother Penelope, have been living at the palace in a riotous three-year-long party. Violating all the accepted rules of the guest-house relationship and ancient courting practices, they have been consuming his household’s food and drink and sleeping with the female servants, trying to force Penelope to give up waiting for Odysseus and marry one of them. Athena is right. Telemachus needs to grow up quickly."