No, there was no tax rate change that would explain this, and your ages don’t matter as far as paying Self-Employment tax goes.
Is there a fairly large amount in line 8a, Total social security wages and tips (total of boxes 3 and 7 on Form(s) W-2) and railroad retirement (tier 1) compensation? As that amount approaches the $142,800 limitation you refer to, you’ll owe less in SE taxes.
Did you actually have W-2 wages that were that high?
