Complex sentence: In 1283, Roger predeceased his wife; when she died in 1292 their son Robert de Clifford, 1st Baron de Clifford was 18 and not old enough to take possession of his lands.
Split into simpler sentences: In 1283, Roger predeceased his wife, who died in 1292. At 18, their son Robert was not old enough to take possession of his lands.