Based on the information from various sources, the recommended level for entering the Underdark in Baldurs Gate 3 is around level 5. This is because the Underdark is known to be quite challenging, with high HP enemies and rough encounters, making it advisable to be adequately prepared before venturing into this area. While it is possible to enter the Underdark at lower levels, it may require more strategic gameplay, stealth, and negotiation to navigate the perils effectively. Additionally, exploring the Underdark at a higher level allows for a more enjoyable and manageable experience, as it provides access to intriguing locations, unique quests, characters, and items. Therefore, players are encouraged to aim for at least level 5 before delving into the Underdark to ensure a smoother and more rewarding gameplay experience.