Many are accustomed to stop eating Matzoh before Pesach. Like everything, there are various customs as far as when to begin our Matzoh withdrawal:

a) Erev Pesach
b) From the beginning of Nissan
c) From right after Purim

I’d like to trace this Minhag from the top down.

The Yerushalmi (Pesachim Chap. 10) states: “Reb Levi said, he who eats Matzoh on Erev Pesach is like one who cohabits with his betrothed in his father in laws house. (commonly understood to mean that he is preemptive and jumping the gun).

The Rambam codifies this (Chametz U’Matzoh 6:12) and it is quoted by Tosfos (Pesachim 99b) as well. Tosfos qualifies the prohibition: only Matzoh which one can fulfill his obligation with at the Seder is proscribed, thus permitting Egg Matzoh or Matzoh made with juice instead of water.

There is some discussion among the Rishonim as to when on Erev Pesach one should stop eating Mazoh. The general consensus among the Poskim is that it begins at Amud Hashachar, (50-72 minutes before sunrise). The Magen Avraham, quoting the Ran, says one should begin the evening prior.

This prohibition is quoted by the Rema (OC 471:2). It’s omission by the Shulchan Aruch is noteworthy; I would be interested in hearing whether Sefardim have a different custom regarding this.

The Mishna Berura quotes a minhag to refrain from Matzoh from Rosh Chodesh Nissan, two weeks prior to Pesach. As we have noted, there is also a common Minhag to abstain from Matzoh from after Purim, or 30 days before Pesach. Rav Moshe Feinstein (OC 1:155) explains these customs:

He deduces from the Yerushalmi quoted above that the reason Chazal don’t want us to eat Matzoh before Pesach is because they want our initial Matzoh ingestion to be as a Mitzvah. At what point would eating Matzoh interfere with Matzoh on Pesach being considered “initial”? Logically this would start from when we begin to prepare for Pesach.

There is a dispute in the Gemara (Pesachim 6a) as to when one should begin Pesach preparations; 30 days or two weeks. So although Chazal didn’t go so far as to prohibit Matzoh from those times, the various customs evolved to refrain from Matzoh from those times.
[For a more in depth discussion of this topic see Igros Moshe Ibid.).

This prohibition does not extend to children too young to comprehend the story of the Exodus.

Matzoh Meal products are included in this prohibition. If, however, they are boiled (such as kneidelach) then one can eat them on erev Pesach. To the best of my knowledge even those who have the extended minhagim cited above do not refrain from eating Matzoh Meal products other than on Erev Pesach proper.