It is very low compared to Avengers first OS Monday but that was April 30, 2012, i.e. the day before May Day, which is a public holiday in many countries. Therefore expect Age of Ultron to make similar numbers (30M+) on Thursday (the eve of May Day) hopefully, before a monster weekend.
EDIT: In fact, Avengers made ~$20M on Wednesday, May 2, 2012 (a non-holiday during its first OS week) which is on par with Age of Ultron's $19M Monday. Nothing to worry about.