The Gemara establishes the case to which you refer where Beis-Din obligated the Noder to pay before he died, in which case an amount must have been specified. Consequently there is neither a doubt regarding the obligation to pay, nor as to the amount.

'Lo Mechusar Umd'na' means that the Beis-Din did not assess the Nidar's property before he died, not his age.

In cases where the defendant's obligation to pay is known, but not the amount, we apply the principle 'ha'Motzi me'Chaveiro, Alav ha'Rayah', and obligate him to pay the minimum sum that we are sure he is Chayav (in the case of Erchin, for example, twenty Shekalim, if we were not sure whether the Nidar had reached the age of twenty or not).

be'Virchas Kol Tuv

Eliezer Chrysler.