Identification is problematic period. Eyewitness ID isn't always reliable.
But I really don't think her ID is what police is going on here.
Because the of what the younger one told police, apparently, the older one was picked up even before mother was able to ID him.
And there is no problem with cross racial identification between younger and older suspects.
Oh, I don't think they're going on her ID much at all either, people were just talking earlier about cross race bias so I tossed in my thoughts. I do think it may be harder to guess the age of some ethnic groups based on how they age. I also think it's harder for old people to identify younger people's age and vice versa. But your point is quite right, witness IDs are problematic in general.