The author is supported by the ETH Zurich and acknowledges the opportunity offered by the editors of JVE to make this review. Discussions with D. Dimov, D. Kostopoulos, A. Kounov, E. Moulas, J. Pleuger, S. Sarov, P. Schenker and D. Seward greatly helped clarifying concepts and correlations. A. Liati offered access to age measurements in press at the time this work was produced.