Are the maternal cousins considered mahram if they raised their orphaned maternal cousin's daughter from a young age?

The children of maternal uncles and aunts, and paternal uncles and aunts, are not considered mahrams. The fact that they raised you or that you lived with them in the same house since childhood does not change this ruling. Therefore, it is not permissible for you to appear before them except with the legal hijab, along with the prohibition of khalwa (private seclusion) and other rulings related to interacting with non-mahram men. This is unless you were suckled by their mother or sister with a legal suckling, in which case they would become mahrams to you, as the Prophet (peace be upon him) said: "What becomes unlawful by lineage becomes unlawful by suckling."