Immigrants to the U.S. are often starting over from scratch. Even highly skilled immigrants such as doctors and lawyers may have to take jobs far below their skill and education levels due to licensure barriers and language limitations. It is unsurprising that immigrants often start at the bottom rung of the wage ladder when they first arrive in the U.S.
A report from the Cato Institute confirms that new immigrants, including those with and without legal status, have wages lower than their native-U.S. born counterparts, but finds that within 20 years, the wage gap diminishes or disappears entirely. The report notes that undocumented immigrants experience a far greater wage penalty, likely due to their lack of legal authorization to work. The report surmises that convergence of immigrant and native-born wages would occur more quickly if undocumented immigrants had a path to legal status.
You can find the report here.