According to the search results, it is recommended to be around level 4-5 before entering the Underdark in Baldurs Gate 3. The enemies in the Underdark are tough, and being at a higher level can make it easier to deal with them. However, it is possible to enter the Underdark at a lower level, but it may require playing more cleverly and depending more on stealth and negotiation. It is important to note that there are multiple entrances to the Underdark, and some are easier to access than others.