Answer:
The difference between AgeRe and DDLG is so simple it's often forgotten: DDLG is a KINK, while AgeRe is NONSEXUAL/SFW/"Safe".
Step-by-step explanation:
DDLG, regardless of whether you are sexual or not, is a kink and is only for people 18+
Age regression is not a kink. Anyone of any age can regress.
DDLG is a lifestyle and/or a relationship dynamic. It is an overall label rather than a single act.
Age regression is an act/ a thing you do.
DDLG can involve regression, but does not have to. Not all littles regress, and do not regress all the time.
DDLG is a kink, regression is not. Some people might have a kink for regression, but regression itself is not. DDLG can involve regression, but regression is its own thing too.