The small plates recordkeepers appear to have been quite old when their recordkeeper sons were born. Concerning these circumstances of age, John Welch says, "the men seemed to live consistently to very old ages, perhaps an indication that they occupied a privileged social status over the common people. The fact that they lived so long is even more remarkable in light of the frequent references in the Small Plates to wars and contentions between the Nephites and the Lamanites, unless these particular people were not generally involved in the military" (see, e.g., Enos 1:14,20; Jarom 1:7; Omni 1:3,10; Words of Mormon 1:13). [John Welch, "Longevity of Book of Mormon People and the 'Age of Man'," from The Journal of Collegium Aesculapium, 1985, 37, reprinted by F.A.R.M.S.]
“I Began to Be Old and an Hundred and Seventy and Nine Years Had Passed Away”
According to Cleon Skousen, in commencing a study of the writings of Enos, we are suddenly confronted with a man who seems to have lived an incredibly long time. Enos says he was alive and serving as prophet and historian of the Nephites clear down to 420 B.C. (assuming Lehi left Jerusalem about 600 B.C.--see Enos 1:25). Jacob, the father of Enos, was born way back around 598 B.C. Then when was Enos born? If he were born when his father, Jacob, was 50 (which would seem to be an extremely conservative estimate), Enos would have lived to the age of 128 before he relinquished the plates in 420 B.C. If we assume that he was born when his father was younger, then the total life span of Enos would have to be extended that much more.
When we come to our Book of Mormon study of the Jaredites (the book of Ether) we will see that it was customary for men who lived a long time to choose one of their younger sons to carry on. This is also true of the early patriarchs. The above circumstances would indicate that Enos was one of the younger sons of Jacob, and that he received the records from Jacob when the latter was very old, perhaps close to a century. [W. Cleon Skousen, Treasures from the Book of Mormon , Vol. 2, p. 2005]
Note* According to the proposed chronology in Appendix A, Jacob was born in the 4th year from when Lehi left Jerusalem. Thus Jacob would have been 97 when he gave the small plates to Enos (Jacob 7:26-27). If Enos was 20 at the time, then he would have been born when Jacob was 77 years old. When "an hundred and seventy and nine years had passed away" shortly before Enos' death, Enos would have been 99. [Alan C. Miner, Personal Notes]