The usual interpretation of this line is to compare it with the previous sledding lines and to comment on the dearth of vitality now that Marie is older. While poetically, Eliot may mean to leave us with the impression that Marie is elderly, in fact the real Marie, Countess Larisch that Eliot met was 30 years older than himself (Eliot's eldest sister was about 20 years older than him). If Eliot met Countess Larisch during his trip to Munich in the summer of 1911, as I suspect, Marie would have been 53. Further, when Marie was spending her winters in the south it was when she was married to Count Larisch. That was when she was between the ages of 19 and 38. She mentions going to Mentone in the French Riviera in her book My Past (pp. 133,357).
Marie divorced the Count in 1896 and remarried in 1897. According to Marie Larisch's book Secrets of a Royal House (p. 217) when she was married to Otto Brucks (1897-1914) they "resided in Munich, winters; Rottach, summers." Rottach-Egern is 54 km south of Munich on the south shore of Tegernsee, a lake.