  We present results of a spectroscopic survey of X-ray- and proper
motion-selected samples of late-type stars in the Lower Cen-Cru (LCC) and Upper
Cen-Lup (UCL) subgroups of the nearest OB association: Sco-Cen. The primary
goals of the survey are to determine the star-formation history of the OB
subgroups, and to assess the frequency of accreting stars in a sample dominated
by ``post-T Tauri'' pre-MS stars. We investigate two samples: (1) ACT and TRC
proper motion candidates with X-ray counterparts in the ROSAT All-Sky Survey
BSC, and (2) G and K-type Hipparcos candidate members (de Zeeuw et al. 1999).
We obtained optical spectra of 130 candidates with the MSSSO 2.3-m DBS. Pre-MS
stars were identified by (1) strong Li 6707A absorption, (2) subgiant surface
gravities, (3) proper motions consistent with Sco-Cen membership, and (4) HRD
positions consistent with being pre-MS. We demonstrate that measuring the
gravity-sensitive band-ratio of Sr II 4077A to Fe I 4071A is a valuable means
of discriminating pre-MS and ZAMS stars. Depending on the choice of
evolutionary tracks, we find the mean pre-MS ages to range between 17-23 Myr
(LCC), and 15-22 Myr (UCL). Accounting for observational uncertainties, it
appears that 95% of the low-mass star-formation in each subgroup occurred in
less than 8 Myr (LCC) and 12 Myr (UCL). We find main sequence turn-off ages for
Hipparcos B-type members to be 16+-1 Myr (LCC) and 17+-1 Myr (UCL). Contrary to
previous findings, it appears that LCC is coeval with, or slightly older than,
UCL. Only 1 out of 110 (0.9^+2.1_-0.8 %) pre-MS solar-type stars in the sample
with ages of 13+-1 (s.e.)+-6 (1sigma) Myr and masses of 1.3+-0.2 Msun shows
both enhanced Halpha emission and a K-band excess indicative of accretion from
a truncated circumstellar disk. (abridged)
