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tokens: are required to find the true convergence time. Figure 6. Simulation of a theoretical 10 Myr old TWA-like association with four different initial velocity dispersions, σ init . The curves show the traceback age that is derived after randomly adding noise that simulates measurement uncertainties. The derived traceback age drops steeply with increasing measured uncertainties. The total measurement uncertainty in our data is dominated by uncertainties in the RV and parallax measurements. See Section 4.1 for more details. Typical expected velocity dispersions for open clusters are around 1–2 km s −1 (Adams 2000 ). Assuming an initial velocity dispersion for TWA of σ init = 2 km s −1 , we would need to measure the UVW velocities with a precision of 0.25 km s −1 to accurately measure the traceback time to within 1 Myr. Given our current mean measurement uncertainties of σ meas = 1.3 km s −1 , the initial velocity dispersion of TWA would have to be greater than 11 km s −1 for us to accurately measure the traceback age to within 1 Myr. The GAIA spacecraft will soon obtain parallaxes to a billion stars with unprecedented precision. Given the low uncertainties in GAIA parallaxes and proper motions, the RVs will become the dominate source of noise. From the calculations above, the RVs will need to have a precision of 0.25 km s −1 for a proper traceback analysis, a quarter of our current average RV uncertainty. We ran another traceback analysis, this time with only stars with RV uncertainties below 0.5 km s −1 (17 stars). This produces a traceback age of 2.9 ± 0.5 Myr. This is a slightly higher value than most of the subsamples tested, but still falls quite short of the expected value. Given the low traceback ages that we obtain for TWA, it is likely that its initial velocity dispersion is too low for an accurate determination of its age using a traceback analysis. 5. PRE-MAIN-SEQUENCE TRACK AGE 5.1. Calculating the Ages of Individual Stars To determine the ages of individual TWA stars, we compare the stars to the pre-main-sequence tracks of Baraffe et al. ( 2015 ). We use our parallax values to convert 2MASS apparent magnitudes to absolute magnitudes. We took optical spectral types from the literature and converted them to effective temperature using Table 6 of Pecaut Mamajek ( 2013 ). For stars M5–M7, we use the main-sequence values in Table 5 of Pecaut Mamajek ( 2013 ). It is interesting to note the disagreement between Pecaut Mamajek ( 2013 ) and Luhman ( 1999 ), which was used in Weinberger et al. ( 2013 ) to convert to effective temperature. The disagreement is typically about 100 K cooler for Pecaut Mamajek ( 2013 ) than for Luhman ( 1999 ), with particular disagreement around M5 with a difference of about 250 K. For stars with spectral type greater than M7, we used the polynomial relation in Table 19 of Faherty et al. ( 2016 ) for moving group members. Spectral types and temperatures for the entire sample are listed in Table 4 . Table 4