  Extremely similar heliocentric orbital elements of the main-belt objects
(458271) 2010 UM26 and 2010 RN221 make them the tightest known pair and promise
its very young age. We analyzed the conditions of its origin and determined its
age. We conducted dedicated observations of (458271) 2010 UM26 and 2010 RN221
in summer 2022 that resulted in a high-accuracy astrometric set of data.
Joining them with the previously available observations, we improved the
precision of the orbit determination of both asteroids. We used numerical
simulations backward in time to constrain the origin of this new pair by
observing orbital convergence in the Cartesian space. Using a large number of
possible clone variants of (458271) 2010 UM26 and 2010 RN221 we find they all
converge in a narrow time interval around March 2003 having extremely tight
minimum distances ($\leq 1000$ km) and minimum relative velocities ($\leq 3$
cm~s$^{-1}$). These conditions require to include mutual gravitational
attraction of the asteroids constituting the pair for its age determination.
Extending our model by this effect even improves the convergence results. We
find there is more than $55$\% probability that the pair formed after the year
2000. However, quasi-satellite captures make the possible age uncertainty of
this pair prolonged possibly to the 1960s. Still, this is by far the youngest
known asteroid pair, a prime target for future astronomical observations.
