In your earlier lessons, when you generated an online horoscope you might have noticed a timeline chart on the page. This is exactly the Vimshottari Dasha (विंशोत्तरी दशा) timeline. For your quick reference, the screen shot of this timeline is shown below:
On the top portion of the chart, the start and end years for each dasha is given. At the bottom, the respective age attained by the patron on that day is shown. Including both date and age makes it easier for you to spell out your predications. For example, rather than saying that in 2044 June, you will have a good fortune, it is better appreciated when you say that you will gain good fortune at the age of 26.
When you examine the above chart, you will notice that from birth till the child becomes 6 yrs/10 months Ketu Mahadasha is prevailing. After Ketu, Venus Mahadasha starts and ends when the patron grows to 26 yrs and 9 mths. The rest of the Mahadashas follow in the pre-defined order. During each mahadasha, the respective planet will generally show better effects of their characteristics. However, this is just a general statement and a deeper analysis is required to do more accurate and better predications.
ps: Many sites give the timeline in a tabular format. I personally prefer the graphical representation of the timeline as it is more intuitive and provides a better visulization.