DS was 22 months - we needed the cot for DD1.
DD1 was 14/15 months - I didn't want her to feel like she was being kicked out when DD2 came along, so we switched her over early.
DD1 was 13 months (well still is 13 months..lol) - DH broke her cot and the older two share a double bed at night, so the toddler bed was free.