At this age, it can be just a nice temperature (7)
I believe the answer is:
seventy
'at' is the definition.
The answer and definition can be both quantities as well as being singular nouns.
Perhaps they are linked in a way I don't understand?
'this age it can be just a nice temperature' is the wordplay.
I cannot really understand how this works, but
'it' could be 't' (abbreviation. e.g. in 'tis) and 't' is present in the answer.
'just' could be 'even' and 'even' is found within the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for seventy that I've seen before include "A number" , "Three score years and ten" , "Threescore and ten" , "LXX" .)