I have a couple of books with build and retirement dates for these units.
The Baldwin (freight) sharks were retired in large blocks, starting on 12/31/63, with the last batch leaving the roster on 1/13/67. As suggested by NorthWest, the retirements were generally within a few months (and in some cases, within a few days) of the units' 15th birthdays.
The Erie Builts followed a similar pattern, with the first retirements on 9/27/62, and the last on 12/30/63. All were approximately 15 years of age at the time of retirement, as well.
I don't know specifically about these units, but I've read that it was a common practice to finance units like this, with 15-year equipment trusts. From a financial perspective, they were easier to dispose when the trusts expired, and they were considered depreciated at that point.
JR