Ms. Johnston, 51, and Mr. Lee, 49, are part of a bumper crop of designers whose sensibilities are bringing new depth to contemporary architecture around the world.

In architecture, where buildings germinate slowly and early commissions tend to be small, Ms. Johnston and Mr. Lee are still young. But after 20 years of practice, they’re seasoned enough for clients like the Menil to entrust them with significant commissions. They are in that fertile period — agewise, it typically runs from the mid-40s to mid-50s in architecture — when the profession’s next generation of leadership begins to make its mark. Mr. Piano was 50 when the Menil, his American debut, opened. It was only after the renovation of his house, at 48, that Mr. Gehry became architecture’s other important Frank. Zaha Hadid was mostly an academic architect well into her 40s.

In 2018, when they complete Rome’s Città del Sole (City of the Sun), Maria Claudia Clemente, 50, and Francesco Isidori, 47, partners of that city’s Labics studio, will have been at work on the complex for most of the time they’ve practiced together. Won in a 2007 competition, the 175,000-square-foot project comprises a library and residential, office, retail, parking and extensive public space — a critical component of Roman urban life. The sleek urban center replaces a derelict former bus depot and repair yard hidden behind fences along Rome’s eastern edge. “But our goal was not to build a collection of interesting architectural objects,” Ms. Clemente and Mr. Isidori said. “We wanted to create a coherent structural tissue for this part of the city.”