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This study investigates the temporal and spatial variations in lithium abundance within the Milky Way using a sample of 22 034 main-sequence turn-off (MSTO) stars and subgiants, characterized by precise stellar ages, 3D non-local thermodynamic equilibrium lithium abundances, and birth radii. Our results reveal a complex variation in lithium abundance with stellar age: a gradual increase from 14 to 6 Gyr, followed by a decline between 6 and 4.5 Gyr, and a rapid increase thereafter. We find that young Li-rich stars (ages |$\lt $| 4 Gyr, A(Li) |$\gt $| 2.7 dex) predominantly originate from the outer disc. By binning the sample according to guiding centre radius and z |$_{\rm max}$|⁠ , we observe that these young Li-rich stars migrate radially to the local and inner discs. In addition, the stars originating from the inner disc experienced a rapid Li enrichment process between 8 and 6 Gyr. Our analysis suggests that the age range of Li-dip stars is 4–5 Gyr, encompassing evolution stages from MSTO stars to subgiants. The Galactic radial profile of A(Li) (with respect to birth radius), as a function of age, reveals three distinct periods: 14–6 Gyr ago, 6–4 Gyr ago, and 4–1 Gyr ago. Initially, the lithium abundance gradient is positive, indicating increasing Li abundance with birth radius. During the second period, it transitions to a negative and broken gradient, mainly affected by Li-dip stars. In the final period, the gradient reverts to a positive trend. [ABSTRACT FROM AUTHOR]

Main-sequence turn-off (MSTO) stars and subgiant stars are good tracers of Galactic populations. We present a study of 41 034 MSTO and subgiant stars from the GALAH survey. Using a grid of stellar models that accounts for the variation of O abundances, we determine their ages with a median age uncertainty of ∼9.4 per cent. Our analysis reveals that the ages of high-O stars based on O-enhanced models are smaller than those determined with α-enhanced models, resulting in a mean fractional age difference of −5.3 per cent at [O/α] = 0.2 and −11.0 per cent at [O/α] = 0.4. This age difference significantly impacts the age distribution of thick disc and halo stars, leading to a steeper downward trend in the [Fe/H]–age plane from 8 to 14 Gyr, indicating a shorter formation time-scale and a faster chemical-enhanced history for these populations. We confirm the V-shape of the normalized age-metallicity distribution p (τ∣[Fe/H]) of thin disc stars, which is presumably a consequence of the second gas infall. Additionally, we find that the halo stars in our sample can be divided into two sequences, a metal-rich sequence (Splash stars) and a metal-poor sequence (accreted stars), with the Splash stars predominantly older than 9 Gyr and the accreted halo stars older than 10 Gyr. Finally, we observe two distinct sequences in the relations between various chemical abundances and ages for disc stars, namely a young sequence with ages < ∼8 Gyr and an old sequence with ages > ∼8 Gyr. [ABSTRACT FROM AUTHOR]