Question: How old is a maple when it is usually tapped?
About the article "Maple syrup", section "Production".

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Maple syrup production is centred in northeastern North America; however, given the correct weather conditions, it can be made wherever suitable species of maple trees grow.
A maple syrup production farm is called a "sugarbush" or "sugarwood".
Sap is often boiled in a "sugar house" (also known as a "sugar shack," "sugar shanty," or "cabane à sucre"), a building louvered at the top to vent the steam from the boiling sap.
Maples are usually tapped beginning at 30 to 40 years of age.
Each tree can support between one and three taps, depending on its trunk diameter.
The average maple tree will produce of sap per season, up to per day.
This is roughly equal to 7% of its total sap.
Seasons last for four to eight weeks, depending on the weather.
During the day, starch stored in the roots for the winter rises through the trunk as sugary sap, allowing it to be tapped.
Sap is not tapped at night because the temperature drop inhibits sap flow, although taps are typically left in place overnight.
Some producers also tap in autumn, though this practice is less common than spring tapping.
Maples can continue to be tapped for sap until they are over 100 years old.
 syrup.
The Sugar maple ("A. saccharum") is tapped for sap, which is then boiled to produce maple syrup or made into maple sugar or maple taffy.
It takes about of sugar maple sap to make of syrup.
While any "Acer" species may be tapped for syrup, many do not have sufficient quantities of sugar to be commercially useful. 
Maple syrup is a syrup usually made from the xylem sap of sugar maple, red maple, or black maple trees, although it can also be made from other maple species.
In cold climates, these trees store starch in their trunks and roots before the winter; the starch is then converted to sugar that rises in the sap in the spring.
Maple trees can be tapped by boring holes into their trunks and collecting the exuded sap.
The sap is processed by heating to evaporate much of the water, leaving the concentrated syrup.
Maple syrup was first collected and used by the indigenous peoples of North America.
The practice was adopted by European settlers, who gradually refined production methods.
Technological improvements in the 1970s further refined syrup processing.
The Canadian province of Quebec is by far the largest producer, responsible for about three-quarters of the world's output; Canadian exports of maple syrup exceed C$145 million (approximately US$130.5 million) per year.
Vermont is the largest producer in the United States, generating about 5.5 percent of the global supply.
Maple syrup is graded according to the Canada, United States, or Vermont scales based on its density and translucency.
Sucrose is the most prevalent sugar in maple syrup.
In Canada, syrups must be at least 66 percent sugar and be made exclusively from maple sap to qualify as maple syrup.
In the United States, a syrup must be made almost entirely from maple sap to be labelled as "maple".
Maple syrup is often eaten with pancakes, waffles, French toast, or oatmeal and porridge.
It is also used as an ingredient in baking, and as a sweetener or flavouring agent.
Culinary experts have praised its unique flavour, although the chemistry responsible is not fully understood. 
In the lumber industry "Acer rubrum" is considered a soft maple.
The wood is close grained and as such it is similar to that of "A. saccharum", but its texture is softer, less dense, and has a poorer figure and machining qualities.
High grades of wood from the red maple can nonetheless be substituted for hard maple, particularly when it comes to making furniture.
As a soft maple, the wood tends to shrink more during the drying process than with the hard maples.
Red maple is also used for the production of maple syrup, though the hard maples "Acer saccharum" (sugar maple) and "Acer nigrum" (black maple) are more commonly utilized.
One study compared the sap and syrup from the sugar maple with those of the red maple, as well as those of the "Acer saccharinum" (silver maple), "Acer negundo" (boxelder), and "Acer platanoides" (Norway maple), and all were found to be equal in sweetness, flavor, and quality.
However, the buds of red maple and other soft maples emerge much earlier in the spring than the sugar maple, and after sprouting chemical makeup of the sap changes, imparting an undesirable flavor to the syrup.
This being the case, red maple can only be tapped for syrup before the buds emerge, making the season very short.
Red maple is a medium quality firewood, possessing lower heat energy, nominally 5.4 MJ/m³ (18.7 million BTU (mbtu) per cord), than other hardwoods such as Ash: 7 MJ/m³ (24 mbtu/cord), Oak: 7 MJ/m³ (24 mbtu/cord), or Birch: 5.8 MJ/m³ (20 mbtu/cord).
Red Maple make vibrant and colorful Bonsai, and have year around attractive features for display. 
Acer saccharinum, commonly known as silver maple, creek maple, silverleaf maple, soft maple, water maple, swamp maple, or white maple—is a species of maple native to eastern North America in the eastern United States and Canada.
It is one of the most common trees in the United States.