  The paper presents magnetic field measurements for 15 chemically peculiar
(CP) stars of subgroup~1b in the OrionOB1 association. We have found that the
proportion of stars with strong magnetic fields among these 15 CP stars is
almost twice as large as in subgroup 1a. Along with this, the age of subgroup
1b is estimated as 2 Myr, and the age of subgroup~1a is in the order of 10 Myr.
The average root-mean-square magnetic field Be for stars in subgroup 1b is 2.3
times higher than that for stars in subgroup 1a. The conclusions obtained fall
within the concept of the fossil origin of large-scale magnetic fields in B and
A stars, but the rate of field weakening with age appears anomalously high. We
present our results as an important observational test for calibrating the
theory of stellar magnetic field formation and evolution.
