So strong is custom formed in early years.

[Adeo in teneris consuescere multum est.]

Virgil (70-19 BC) Roman poet [b. Publius Vergilius Maro; also Vergil]

Georgics [Georgica], Book 2, l. 272ff (2.272) (29 BC) [tr. Greenough (1900)]
    (Source)

Discussing how, when transplanting vines, wise farmers try to match the soil and orientation of the plant toward the sun to the conditions where they first sprouted. The same phrase is often extended (when extracted like this) to the lasting effects of early training on children. See also Pope.

(Source (Latin)). Alternate translations:

Such strength hath custome in each tender soule.
[tr. Ogilby (1649)]

So strong is Custom; such Effects can Use
In tender Souls of pliant Plants produce.
[tr. Dryden (1709), ll. 366-367]

So strong is habit's force in tender age.
[tr. Nevile (1767), l. 302]

So custom strongly sways the youthful year.
[tr. Sotheby (1800)]

Of such avail is custom in tender years.
[tr. Davidson (1854)]

So custom lords it o'er the youthful wood.
[tr. Blackmore (1871), l. 324]

Such is the force of habits formed in early years.
[tr. Wilkins (1873)]

So strong is custom formed in early years.
[tr. Rhoades (1881)]

So powerful is habit in things of tender age.
[tr. Bryce (1897)]

So strong is the habit of infancy.
[tr. Mackail (1899)]

So potent is early habit's control.
[tr. Way (1912)]

So loth to change
Are a young creature's ways.
[tr. Williams (1915)]

So strong is habit in tender years.
[tr. Fairclough (Loeb) (1916)]

So important are habits developed in early days.
[tr. Day-Lewis (1940)]

For habit dominates the early stage.
[tr. Bovie (1956)]

So much effect has habit on the young.
[tr. Wilkinson (1982)]

We grow accustomed to so much in tender years.
[tr. Kline (2001)]

How powerful the innate habits of tender plants!
[tr. Lembke (2004)]

So powerfully runs habit in the tender stems.
[tr. Johnson (2009)]

Such is the need, when young, of what's familiar.
[tr. Ferry (2015)]