To state bluntly, there is no such thing as a sub-adult accipiter. Accipiters show either a "juvenile" (1st-year) plumage or "adult" (2nd-year and older) plumage (this is also true for most raptors, i.e. Red-tailed Hawk). The adult plumage is acquired after their first molt, which starts at about one year old and takes several months to complete. Some birds retain a few juvenile feathers after their first molt, but these birds are not "sub-adults". They are adults, since they are in adult plumage. Besides, there are many birds that don't retain any (or any obvious) juvenile feathers in their first adult plumage. A juvenile in summer going through its' first molt that has a mix of adult and juvenile feathers is still not a sub-adult. They are birds in "transition" or "molting juveniles". A true sub-adult raptor shows real and certain plumage differences from juvenile and adult. They may not appear completely different, but have basic differences that are diagnostic. For instance, Swainson's Hawk has a sub-adult plumage that always differs from juvenile and adult (albeit, not greatly).
Be very careful of telling retained juvenile from retained (old and faded) adult feathers, I would be wary of distinguishing these without practice! Knowing molt patterns and sequences helps greatly in confirming ages based on the presence of retained feathers. Eye color can be an indicator of age but not a criteria when used alone for ageing raptors. Eye color usually changes quicker in males than females or between individuals. And eye color tone and rate of change varies between buteo species. For instance, Swainson's, Broad-winged, Red-shouldered, and Rough-legged Hawks tend to turn from pale to dark brown quicker than Red-tailed Hawks...and Ferruginous Hawks take quite a bit longer than all of them.
This "sub-adult" issue has come up more recently due to all the close-up photos on the internet, but seeing this stuff in the field or in flight is much more difficult, and sometimes impossible. Take a look at the birds below, some have a few retained juvenile feathers, some have none, some have 2 generations of adult feathers, some have pale eyes, and others have darker eyes. Would you be able to age these? How about in flight? The particular age of each bird below is less important than the fact that they are all in adult plumage.
And, one question: Why does the term "sub-adult" seems to be used frequently on the internet for accipiters, but no one seems to call a Red-tail with a few retained juvenile feathers a "sub-adult?" I know why...