Determining the composition of giant exoplanets is crucial for understanding their origin and evolution. However, planetary bulk composition is not measured directly but must be deduced from a combination of mass-radius measurements, knowledge of the planetary age and evolution simulations. Accurate determinations of stellar ages, mass-radius measurements, and atmospheric compositions from upcoming missions can significantly improve the determination of the heavy-element mass in giant planets. In this paper we first demonstrate the importance of an accurate age measurement, as expected from Plato, in constraining planetary properties. Well-determined stellar ages can reduce the bulk-metallicity uncertainty by up to a factor of two. We next infer the bulk metallicity of warm giants from the Ariel mission reference sample and identify the Ariel high-priority targets for which a measured atmospheric metallicity can clearly break the degeneracy in the inferred composition. We show that knowledge of the atmospheric metallicity can broadly reduce the bulk-metallicity uncertainty by a factor of four to eight. We conclude that accurate age determinations from Plato and atmospheric measurements by Ariel and the James Webb Space Telescope will play a key role in revealing the composition of giant exoplanets.