I got the sense that Dru was supposed to be younger when sired. Late teens/early 20s. Whereas Spike seemed more like late 20s/early 30s. But that’s just my impression. I don’t think we have actual canon ages.
I'd say she was sired early 20s and Spike late 20s/early 30s.
Assuming Spike was 30 in 1880, that would put his birth year as 1850. Dru was sired in 1860 which would put her birth year as 1840, making her 10 years older than Spike.
Drusilla still seemed reasonably young and innocent in the flashback in Becoming. Spike in the flashbacks of LMPTM, and consequently FFL, seemed like he was old enough that he should've been married by then instead of being the manchild that he was. Drusilla was very unimpressed by his suggestion of bringing his mother along on their travels.