Tenant Screening - TransUnion says possible invalid SSN

[Joe P.]

Top applicant for a property appears to be a mother-son combo, he is 23 and speaks good English. Both are from Columbia and appear to have come here in the last year or two. Both have SSN, hard-working, blue-collar, very kind people. Glowing report from current landlord, all income verified, no criminality, bad credit but mostly because there isn't enough history.

TransUnion's only real flag (besides the credit) is that SSN may be invalid, and I'm not sure how to take it. Should I/can I do further checks on this? Is it because the SSN may have been issued recently?