This is a really interesting question, and the answer is certainly not just "description." No. In fact, most of Hagar's extreme old age (and depreciation due to the sin of pride) comes from Laurence's use of both flashback and/or memories that always lead to a philosophical discussion.
Hagar proves her life, in all stages, to be a compete land of desolation. Yes, she is described as ninety years old, but that is only the beginning. What has aged her is her pride.
Too bad to deprive them, but if a person doesn't look after herself in this world, no one else is likely to.
Hagar reveals this through a series of vignettes that are both flashbacks and memories. For example, in her young life Hagar denies her father's blessing on a marriage because she simply feels passion for a young man who is dancing (Bram Shipley). She drinks wine with tramps in abandoned buildings, runs away from finishing schools, ducks from her obligations as mother, and she eventually lies to her son on her deathbed. It is bit redeeming, though, that Hagar admits her pride at the end.
In conclusion, it's safe to say that both memory and flashback reveal the most about Hagar's extreme old age. It is in this way (and with this vivid style) that Laurence makes Hagar such a striking character in the world of Literature.