Would think they are last springs youngsters possibly not as well grown as they should be by this time possibly reared by one parent, while there is quite a difference as to when cubs are born depending on which part of the country they are in there are certain things that make it unlikely that these are young cubs.
In the south cubs may be born early in the year February is not unusual and even earlier on occasion, up here
Cubs tend to be born late April or even May.
One thing remains constant nature engineers that cubs are born to coincide with with the birth of other creatures ie
spring.
While a lone adult fox will survive and thrive on carrion and what he can catch, a litter of cubs need a lot of food to support them and a litter born late in the year as you suggest these have been , would be very unlikely
to survive
Down to the difficulty the parents would have in finding enough food to support them.
Gestation period for foxes is about ten days less than dogs typically 52 53 days if these were young cubs as you suggest it would mean that the vixen came into heat sometime in September which is highly unlikely , and as I have already said even if that was the case however unlikely she would still have been unlikely to rear them
So think its much more likely that they are cubs from last spring.