In the Bible children were children from 0-19 years old and were in the promised land. At 20 they were put out into the wilderness (Numbers 14:29-33) (Deu 1:39) (Exodus 30:14) -at 20 they became fully accountable adults before God. Though, it seems that it was not until 30 that they were considered mature adults. Priests were in training up until 30 and Jesus didn’t begin his ministry until about 30 (Luke 3:23-38) as Jews didn’t consider adults fully mature until 30. From 30-50 you could serve in the ministry (Numbers 4:30) and were under the oversight of church elders over 50. Levites at 50 were to retire from active duty and minister/oversee those below this age (Numbers 8:25). Although 40 is symbolically important in the Bible, it though doesn’t seem to have the age significance that the modern world has put on it. It just seems to mean lots of years, two generations.
A 40 year period of probation, trials and tribulations followed by renewal. Considering that children were in the promised land until 20 and were sent out for 40 years, 60 would suggest when life should “begin” and not 40. Any age under 60 is referred to as “young” in the Bible (1 Timothy 5:9), one score is twenty years and three score is 60 years.
40 years can also be a time for deepened reflection and movement. Moses was on probation for 40 years - the first twenty years of his life marked uncertainty and approaching 40/ a full forty years he left Egypt and decided to visit his own people Acts 7:23 and brought his people out of Egypt. This was God’s time for Moses. When Rehoboam started to reign at age 41, he was considered to be a young man 1Kings 14:21 and 1 Kings 12:8-16 and so to was Joshua at 50 considered young Exodus 33:11. It was Joshua who led the Israelites called by God into their promised land, the land of Canaan which was later to became known as Israel (Joshua 1-4). It was Jacob at a great age (see below) who fulfiled this passage of change name from Canaan to Israel.
So, in Biblical terms we know that 0-19 is childhood, 20-29 is a young adult. 30-49 is mature adulthood, 50-59 is not old but suggests that a greater level of understanding and maturity has been reached. 60-79 is old age with the Bible suggesting a phase I from 60-69 and phase II from 70-79 and it’s also mentioned in the Bible that 80+ is “very old” or of “great age” (2 Samuel 19:32) (Luke 2:36-38). Prophetess Anna who was widowed served in the Temple at 87 years of age in the New Testatment and didn't linger on her past. Middle age is a modern day construct. In the Bible a full maxium life span is given at 120 years of age which 40 is just a quarter of (Genesis 6:3). Some people today are still blessed to be able reach such great milestones.