The time lag design has been defined by Schaie (1965) as examining "whether there are differences in a give characteristic for samples of equal age but drawn from different cohorts measured at different times" (p. 95). In other words, only one age is studied but across different cohorts at different times.

The time lag design could also be defined by Cook and Campbell (1979), as a separate sample design. As such, it also confounded by differences in generations or cohorts. According to Schaie (1970), the time lag method is designed to measure cultural change but confounds environmental treatments or normative history-graded influences with differences between cohorts.

In simple words, this research method involves the comparison of people that are the same age, but from different cohorts. An example of this would be to analyze differences from 20 year olds that lived in the 70s, 80s and 90s. Strengths of this method are that it controls for cohort differences. Weaknesses are that this method is very expensive and you cannot control for age change effects or age differences.

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Sources:
http://webcache.googleusercontent.com/search?q=cache:TqC4X82g68UJ:www2.webster.edu/~woolflm/designs.html+&cd=2&hl=en&ct=clnk&gl=in
http://quizlet.com/4228788/psych-230-developmental-psychology-exam-1-flash-cards/