When does a pond become a lake?

From New Hampshire -- Probably varies by location:

From a regulatory viewpoint there is no distinction between a lake and a pond.

The term "lake" or "pond" as part of a waterbody name is arbitrary and not based on any specific naming convention. In general, lakes tend to be larger and/or deeper than ponds, but numerous examples exist of "ponds" that are larger and deeper than "lakes." For example, Echo "Lake" in
Conway is 14 acres in surface area with a maximum depth of 11 feet, while Island "Pond" in Derry is nearly 500 acres and 80 feet deep. Names for lakes and ponds generally originated from the early settlers living near them, and the use of the terms "lake" and "pond" was completely
arbitrary. Many have changed names through the years, often changing from a pond to a lake with no change in size or depth. Often these changes in name were to make the area sound more attractive to perspective home buyers. Examples of ponds that are now called lakes include Mud Pond to Mirror Lake in Canaan, Mosquito Pond to Crystal Lake in Manchester and Dishwater Pond to Mirror Lake in Tuftonboro.

Original source: des.nh.gov

It is arbitrary.

The only clear cut distinction between the two is that a lake is always a natural formation; if it is manmade, the body is classified as a pond. Ponds are created by farmers to provide water for livestock. Some ponds are created to provide feeding and nesting for waterfowl. Hatcheries create stocked ponds to breed fish.

https://en.wikipedia.org/wiki/Pond

The answer to this must depend on which side of "the pond" (Atlantic Pond!) one lives. On the west side, lots of ponds have names, even though they are generally (but not always) smaller than lakes.

*Thoreau, who I mentioned above, built a cabin on the shores of Walden Pond.
*Henry Fonda finally won an Oscar for "On Golden Pond," a named but fictitious pond. The screenwriter and playwright lived on Great Pond, one of over two dozen named ponds in Kennebec County, Maine. We have hundreds of named ponds.

https://en.wikipedia.org/wiki/On_Golden_Pond_%281981_film%29#Production_notes

We still call the natural pond on our property "the pond," although we should start calling it "the marsh," and the one scooped out with a bulldozer to catch meltwater for irrigation we call "the farm pond."

There is considerable uncertainty about defining the difference between lakes and ponds, and no current internationally accepted definition of either term across scientific disciplines or political boundaries exists.

For example, limnologists have defined lakes as water bodies which are simply a larger version of a pond, which have wave action on the shoreline or where wind-induced turbulence plays a major role in mixing the water column. None of these definitions completely excludes ponds and all are difficult to measure. For this reason there has been increasing use made of simple size-based definitions to separate ponds and lakes. One definition of lake is a body of water of 2 hectares (5 acres) or more in area, however others have defined lakes as waterbodies of 5 hectares (12 acres) and above, or 8 hectares (20 acres) and above (see also the definition of "pond"). Charles Elton, one of the founders of ecology, regarded lakes as waterbodies of 40 hectares (99 acres) or more.

https://en.wikipedia.org/wiki/Lake

It seems like a "pond" larger than 8 hectares is a "lake".

https://en.wikipedia.org/wiki/Pond#Technical_definitions

According to this convention, that is:

https://en.wikipedia.org/wiki/Ramsar_Convention

It does not appear there is a set definition, although the world's smallest lake is 15 square meters.

http://www.chinadaily.com.cn/m/liaoning/2010-07/19/content_10125175.htm

Here's an official definition which may explain the Chinese example:

If the water is deep enough that light does not penetrate to the bottom, and photosynthesis is limited to the top layer, the body of water is considered a lake. A pond is a body of water shallow enough to support rooted plants. Many times plants grow all the way across a shallow pond.

http://mentalfloss.com/article/57902/whats-difference-between-lake-and-pond (satguru)

http://www.funtrivia.com/playquiz/quiz2348831ae4a70.html

User Cymruambyth attempts to answer the question with his/her quiz devoted to lakes, ponds, and streams.

Take the quiz and you will discover the answer. I don't want to post it here to compromise the quiz for those who choose to take it. But I will include the reasoning, so if you plan on taking the quiz don't read beyond this line:

____

In the U.S. a pond is defined as a body of water with a surface area of less than ten acres (40,000 square metres). By that definition, a puddle could be classed as a pond. On the other side of the 'Big Pond' (aka the Atlantic Ocean), in some parts of England, a pond is only called a pond if it is man-made. The international definition is that a pond is a body of water into the deepest areas of which sunlight can reach. Some ponds are made by human hands (when our ancestors first began domesticating animals like sheep and cows, for instance, they created dew ponds to provide water for the stock, and we've all admired those lovely Japanese style artificial ponds stocked with beautiful koi). Some ponds are ancient, left behind as the glaciers receded. These are known to geologists as 'kettle ponds'. Thoreau's famous Walden is a kettle pond. Most ponds, however, are the result of rain run-off, small springs or small streams, while lakes are usually fed by rivers, creeks and/or springs. As to the choice that suggested that lakes support life and ponds do not, that's just silly. Mosquito larvae can grow to maturity in ditches and bird baths, so it follows that any body of water (with the exception of the overly-saline waters like the Dead Sea) can support life of one form or another.