Hi, Dr. Fournier and others.
I made some progress. Exploring many possibilities, I found that the following assumptions help, but there is still a concern.
(Assumption 1) Three age classes are not from the same cohort, and thus age class 1 and 3 are presumed to be independent. Thus I fix the correlation coefficient between age class 1 and 3 as zero.
(Assumption 2) If there is a correlation between neighbor age classes (e.g., "1 vs. 2", and "2 vs. 3"), the correlation should be positive (i.e. autocorrelation over the years). Thus I set bounds of correlation between those neighbor classes to be positive: (e.g., init_bounded_number corp12(0.0,0.7,3); )
Then now I have the resultant COR file (i.e., convergence was made). It was good news, but the estimates of correlation coefficients between neighbor age classes are stuck to the uppder bound, 0.7. If I allow a wide bound (e.g., init_bounded_number corp12(0.0,0.9,3); ), then I failed to get positive definite matrices.