Complex sentence: At the age of at least 15 years, Dahl had to decide where he was going to study art, though the only options in Sweden at the time were an apprenticeship in the Painters' Guild or joining the Martin Hannibal and David Klöcker Ehrenstrahl group.
Split into simpler sentences: At the age of at least 15 years, Dahl had to decide where he was going to study art, though it wasn't much of a choice. The only options in Sweden at the time were becoming an apprentice in the Painters' Guild or joining the Martin Hannibal and David Klöcker Ehrenstrahl group.