Surprisingly , current atmospheric models suggest that the coolest T dwarfs ( T8.5 to T10 ) are young and very low mass ( 0.06 - 2 Gyr , 5 - 20 M _ { jup } , Leggett et al . 7 , 2010 , 2012 ) .
Studies of population kinematics offer an independent constraint on the age of the population .
We present kinematic data of a sample of 75 mid to late T dwarfs drawn from a variety of sources .
We define our samples , T5.5 to T8 and T8.5 to T10 , as mid and late T respectively .
UKIDSS LAS kinematics were derived from our automated LAS proper motion pipeline and distance estimates derived from spectral types and photometry for the minority of sources that lack parallaxes .
Our results show that the mid and late T populations do not have distinctly separate tangential velocity distributions to 95 % probability .
They also give an approximate mean kinematic age equal to that of a population with B - V colour 0.51 - 0.54 , and a spectral type late F , which corresponds to an age of about 2 Gyr .
However the median and modal ages are greater .
This indicates that while model atmospheres correctly predict some trends in colour with gravity and age , reliable ages can not yet be inferred from them .
More benchmark objects are needed to anchor the models .