I am a mechanical engineer, and I haven't seen the lack of Caucasian Americans under 30 for our company. However, the number of AAs in mechanical engineering is quite low.
Most of the folks I know who are Mechanical Engineers work for firms which are involved with HVAC, refrigeration, etc. They all have their own businesses and the work is intensely hands-on and outdoors. According to several of these friends, it is next to impossible to find young folks in the AA community who want to start on the bottom and work their way up in a profession like that.
After trying to recruit in the AA community for 15 years now(since his son graduated HS and started working with him while attending college), he has finally given up and started hiring a number of Hispanics who are not afraid of hard outdoor work but have poor written and verbal language skills. Designing and installing Geothermal Heat Pumps for small businesses and private homes is the work that keeps him busy now.