Transposons and the net amount of DNA
Transposition may occur at any given time in life, and in any cell. There are several cases known where transposition interests somatic cells (give a look here for reference). However, the frequency of transposition events is low, and the difference in amount of DNA between different cells in an individual is low. When you reason at the level of different individuals, however, the proportion of genome affected by transposable elements may be relatively high (give a look here for a reference)