OOP: Sirius and his mother--ages

[Theodore Harvey]

No: HPFGUIDX 64465

I haven't posted since I first joined the group quite awhile ago, 
but I've really enjoyed reading the comments on OoP and hope 
to become more active.  I thought I'd mention a picky detail about 
Sirius and his mother that bothered me.  We know from various 
clues that Sirius is a fairly young man--the Lexicon estimates b. 
1960, so 35-36 in book 5.  OoP describes his mother, who died 
about ten years ago (i.e. c. 1985), as an old woman.  But if Sirius 
was really only about 25 when his mother died, how could she 
have been as old as the description of her portrait would imply? 
Are witches capable of having children at a much later age than 
Muggles?  I just wondered if anyone had any thoughts on this.