Samwise Gamgee encountered Robin Smallburrow in November III 3019, and at that point Robin said that he had become a Shirriff seven years earlier (or six years before the War). If we assume that he joined the Shirriffs when he came of age (thirty-three in the Shire), then he would have been born in III 2979 (or 1379 by the Shire-reckoning). This is necessarily approximate, but it suggests that Robin would have been about the same age as Merry Brandybuck, who was born in III 2982 (SR 1382).