Complex sentence: The title "Master" is much less frequently used than formerly, though it is sometimes still used as the written form of address for boys below some undefined age, often regarded as about 13 in formal correspondence, particularly invitations to formal events.
Split into simpler sentences: The title "Master" is much less frequently used than formerly. Master is still sometimes used as the written form of address for boys of below some undefined age, often regarded as about 13 in formal correspondence, particularly invitations to formal events.