I am sure your Allans are related to mine. However I don't think Robert Allan was the youngest. My gr gr grandmother was Barbara Allan. Her brother was Robert Allan. In the 1841 Census at Rayne, Aberdeenshire Robert (35) is living with mother Isobel (65) nb - later census spelt Isabella. In 1841 cens. Robert (46) married to Jean Allan (34);they have 4 chn: Robert (8); George (7); Isabella (4) and William (2). Also Robert's mother Isabella (age 77!),Barbara Allan (40)and Barbara's son Robert Garden (4). Robert is my great grandfather and from being a cattle boy ended up going to Orkney in 1873. He became the wealthiest trader of Orkney & north Scotland (and the greatest benefactor). He was known as the Merchant Prince of Orkney and LOTS has been written about him. His mother, Barbara, went to live with him in Orkney and her death certificate d. 1890. aged 79 lists her father as James Allan a crofter. Barbara never married John Garden the father of her child and in 1840 he married a Jean Lyon with whom he had one child. In 1850 he drowned. At Robert Garden's christening in 1846 one of the witnesses include Robert Allan.
I have not been able to work out what happened to Robert and Jean Allen after 1851 and can not find them listed on any Scottish censuses.