What does the letter "Lām" mean in "Teach your children the prayer at [the age of] seven"?

The lam in the hadith "Command your children to pray at seven years of age," and in similar expressions like "I wrote it when five [days/years] remained," "He was cupped at seventeen," and "The Messenger of Allah (peace and blessings be upon him) married me at seven years of age," means "at" or "when."